gdb: work around prompt corruption caused by bracketed-paste-mode

In this commit:

  commit b4f26d54
  Date:   Tue Mar 2 13:42:37 2021 -0700

      Import GNU Readline 8.1

We imported readline 8.1 into GDB.  As a consequence bug PR cli/28833
was reported.  This bug spotted that, when the user terminated GDB by
sending EOF (usually bound to Ctrl+d), the last prompt would become
corrupted.  Here's what happens, the user is sat at a prompt like
this:

  (gdb)

And then the user sends EOF (Ctrl+d), we now see this:

  quit)
  ... gdb terminates, and we return to the shell ...

Notice the 'quit' was printed over the prompt.

This problem is a result of readline 8.1 enabling bracketed paste mode
by default.  This problem is present in readline 8.0 too, but in that
version of readline bracketed paste mode is off by default, so a user
will not see the bug unless they specifically enable the feature.

Bracketed paste mode is available in readline 7.0 too, but the bug
is not present in this version of readline, see below for why.

What causes this problem is how readline disables bracketed paste
mode.  Bracketed paste mode is a terminal feature that is enabled and
disabled by readline emitting a specific escape sequence.  The problem
for GDB is that the escape sequence to disable bracketed paste mode
includes a '\r' character at the end, see this thread for more
details:

  https://lists.gnu.org/archive/html/bug-bash/2018-01/msg00097.html

The change to add the '\r' character to the escape sequence used to
disable bracketed paste mode was introduced between readline 7.0 and
readline 8.0, this is why the bug would not occur when using older
versions of readline (note: I don't know if its even possible to build
GDB using readline 7.0.  That really isn't important, I'm just
documenting the history of this issue).

So, the escape sequence to disable bracketed paste mode is emitted
from the readline function rl_deprep_terminal, this is called after
the user has entered a complete command and pressed return, or, if the
user sends EOF.

However, these two cases are slightly different.  In the first case,
when the user has entered a command and pressed return, the cursor
will have moved to the next, empty, line, before readline emits the
escape sequence to leave bracketed paste mode.  The final '\r'
character moves the cursor back to the beginning of this empty line,
which is harmless.

For the EOF case though, this is not what happens.  Instead, the
escape sequence to leave bracketed paste mode is emitted on the same
line as the prompt.  The final '\r' moves the cursor back to the start
of the prompt line.  This leaves us ready to override the prompt.

It is worth noting, that this is not the intended behaviour of
readline, in rl_deprep_terminal, readline should emit a '\n' character
when EOF is seen.  However, due to a bug in readline this does not
happen (the _rl_eof_found flag is never set).  This is the first
readline bug that effects GDB.

GDB prints the 'quit' message from command_line_handler (in
event-top.c), this function is called (indirectly) from readline to
process the complete command line, but also in the EOF case (in which
case the command line is set to nullptr).  As this is part of the
callback to process a complete command, this is called after readline
has disabled bracketed paste mode (by calling rl_deprep_terminal).

And so, when bracketed paste mode is in use, rl_deprep_terminal leaves
the cursor at the start of the prompt line (in the EOF case), and
command_line_handler then prints 'quit', which overwrites the prompt.

The solution to this problem is to print the 'quit' message earlier,
before rl_deprep_terminal is called.  This is easy to do by using the
rl_deprep_term_function hook.  It is this hook that usually calls
rl_deprep_terminal, however, if we replace this with a new function,
we can print the 'quit' string, and then call rl_deprep_terminal
ourselves.  This allows the 'quit' to be printed before
rl_deprep_terminal is called.

The problem here is that there is no way in rl_deprep_terminal to know
if readline is processing EOF or not, and as a result, we don't know
when we should print 'quit'.  This is the second readline bug that
effects GDB.

Both of these readline issues are discussed in this thread:

  https://lists.gnu.org/archive/html/bug-readline/2022-02/msg00021.html

The result of that thread was that readline was patched to address
both of these issues.

Now it should be easy to backport the readline fix to GDB's in tree
copy of readline, and then change GDB to make use of these fixes to
correctly print the 'quit' string.

However, we are just about to branch GDB 12, and there is concern from
some that changing readline this close to a new release is a risky
idea, see this thread:

  https://sourceware.org/pipermail/gdb-patches/2022-March/186391.html

So, this commit doesn't change readline at all.  Instead, this commit
is the smallest possible GDB change in order to avoid the prompt
corruption.

In this commit I change GDB to print the 'quit' string on the line
after the prompt, but only when bracketed paste mode is on.  This
avoids the overwriting issue, the user sees this:

  (gdb)
  quit
  ... gdb terminates, and returns to the shell ...

This isn't ideal, but is better than the existing behaviour.  After
GDB 12 has branched, we can backport the readline fix, and apply a
real fix to GDB.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28833