gdb: extend completion of quoted filenames to work in brkchars phase

Up to this point filename completion for possibly quoted filenames has
always been handled during the second (non-brkchars) phase of
completion.  This works fine for commands that only want to complete
on a single filename argument.

In a later commit though I need to perform completion of a quoted
filename argument during the first (brkchars) phase of completion.
This will allow me to add a custom completer that completes both
command options and arguments for a command (remove-symbol-file) that
takes a possibly quoted filename argument.

This commit doesn't add the remove-symbol-file completer, this commit
is just about putting support for that in place.

To achieve this I've added the new function
advance_to_filename_maybe_quoted_complete_word_point, which is unused
in this commit.  I've then had to extend some other functions in order
to extract the quoting state during the brkchars phase.

As this commit doesn't use the new functionality, the important thing
at this point is that I've not regressed the existing filename
completion (or any of the other completion).  The next commit in this
series will make use of the new functionality, and will include
tests.

There should be no user visible changes after this commit.