gdb: handle bracketed-paste-mode and EOF correctly

gdb: handle bracketed-paste-mode and EOF correctly

This commit replaces an earlier commit that worked around the issues
reported in bug PR gdb/28833.

The previous commit just implemented a work around in order to avoid
the worst results of the bug, but was not a complete solution.  The
full solution was considered too risky to merge close to branching GDB
12.  This improved fix has been applied after GDB 12 branched.  See
this thread for more details:

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

This commit replaces this earlier commit:

  commit 74a159a420d4b466cc81061c16d444568e36740c
  Date:   Fri Mar 11 14:44:03 2022 +0000

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

Please read that commit for a full description of the bug, and why is
occurs.

In this commit I extend GDB to use readline's rl_deprep_term_function
hook to call a new function gdb_rl_deprep_term_function.  From this
new function we can now print the 'quit' message, this replaces the
old printing of 'quit' in command_line_handler.  Of course, we only
print 'quit' in gdb_rl_deprep_term_function when we are handling EOF,
but thanks to the previous commit (to readline) we now know when this
is.

There are two aspects of this commit that are worth further
discussion, the first is in the new gdb_rl_deprep_term_function
function.  In here I have used a scoped_restore_tmpl to disable the
readline global variable rl_eof_found.

The reason for this is that, in rl_deprep_terminal, readline will
print an extra '\n' character before printing the escape sequence to
leave bracketed paste mode.  You might then think that in the
gdb_rl_deprep_term_function function, we could simply print "quit" and
rely on rl_deprep_terminal to print the trailing '\n'.  However,
rl_deprep_terminal only prints the '\n' when bracketed paste mode is
on.  If the user has turned this feature off, no '\n' is printed.
This means that in gdb_rl_deprep_term_function we need to print
"quit" when bracketed paste mode is on, and "quit\n" when bracketed
paste mode is off.

We could absolutely do that, no problem, but given we know how
rl_deprep_terminal is implemented, it's easier (I think) to just
temporarily clear rl_eof_found, this prevents the '\n' being printed
from rl_deprep_terminal, and so in gdb_rl_deprep_term_function, we can
now always print "quit\n" and this works for all cases.

The second issue that should be discussed is backwards compatibility
with older versions of readline.  GDB can be built against the system
readline, which might be older than the version contained within GDB's
tree.  If this is the case then the system readline might not contain
the fixes needed to support correctly printing the 'quit' string.

To handle this situation I have retained the existing code in
command_line_handler for printing 'quit', however, this code is only
used now if the version of readline we are using doesn't not include
the required fixes.  And so, if a user is using an older version of
readline, and they have bracketed paste mode on, then they will see
the 'quit' sting printed on the line below the prompt, like this:

  (gdb)
  quit

I think this is the best we can do when someone builds GDB against an
older version of readline.

Using a newer version of readline, or the patched version of readline
that is in-tree, will now give a result like this in all cases:

  (gdb) quit

Which is what we want.

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