gdb: improve gdb_rl_find_completion_word for quoted words

The function gdb_rl_find_completion_word is very similar to the
readline function _rl_find_completion_word, but was either an older
version of that function, or was trimmed when copying to remove code
which was considered unnecessary.

We maintain this copy because the _rl_find_completion_word function is
not part of the public readline API, and we need to replicate the
functionality of that function as part of the 'complete' command.

Within gdb_rl_find_completion_word when looking for the completion
word, if we don't find a unclosed quoted string (which would become
the completion word) then we scan backwards looking for a word break
character.  For example, given:

  (gdb) complete file /tmp/foo

There is no unclosed quoted string so we end up scanning backwards
from the end looking for a word break character.  In this case the
space after 'file' and before '/tmp/foo' is found, so '/tmp/foo'
becomes the completion word.

However, given this:

  (gdb) complete file /tmp/foo\"

There is still no unclosed quoted string, however, when we can
backwards the '"' (double quotes) are treated as a word break
character, and so we end up using the empty string as the completion
word.

The readline function _rl_find_completion_word avoids this mistake by
using the rl_char_is_quoted_p hook.  This function will return true
for the double quote character as it is preceded by a backslash.  An
earlier commit in this series supplied a rl_char_is_quoted_p function
for the filename completion case, however, gdb_rl_find_completion_word
doesn't call rl_char_is_quoted_p so this doesn't help for the
'complete' case.

In this commit I've copied the code to call rl_char_is_quoted_p from
_rl_find_completion_word into gdb_rl_find_completion_word.

This half solves the problem.  In the case:

  (gdb) complete file /tmp/foo\"

We do now try to complete on the string '/tmp/foo\"', however, when we
reach filename_completer we call back into readline to actually
perform filename completion.  However, at this point the WORD variable
points to a string that still contains the backslash.  The backslash
isn't part of the actual filename, that's just an escape character.

Our expectation is that readline will remove the backslash when
looking for matching filenames.  However, readline contains an
optimisation to avoid unnecessary work trying to remove escape
characters.

The readline variable rl_completion_found_quote is set in the readline
function gen_completion_matches before the generation of completion
matches.  This variable is set to true (non-zero) if there is (or
might be) escape characters within the completion word.

The function rl_filename_completion_function, which generates the
filename matches, only removes escape characters when
rl_completion_found_quote is true.  When GDB generates completions
through readline (e.g. tab completion) then rl_completion_found_quote
is set correctly.

But when we use the 'complete' command we don't pass through readline,
and so gen_completion_matches is never called and
rl_completion_found_quote is not set.  In this case when we call
rl_filename_completion_function readline doesn't remove the escapes
from the completion word, and so in our case above, readline looks for
completions of the exact filename '/tmp/foo\"', that is, the filename
including the backslash.

To work around this problem I've added a new flag to our function
gdb_rl_find_completion_word which is set true when we find any quoting
or escaping.  This matches what readline does.

Then in the 'complete' function we can set rl_completion_found_quote
prior to generating completion matches.

With this done the 'complete' command now works correctly when trying
to complete filenames that contain escaped word break characters.  The
tests have been updated accordingly.