[Bug cli/28833] Ctrl-D distorts GDB prompt

["aburgess at redhat dot com" <sourceware-bugzilla@sourceware.org>]

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

--- Comment #7 from Andrew Burgess <aburgess at redhat dot com> ---
Indeed.  The problem seems to be that we are not correctly using readline's
callback interface.

In event-top.c:command_line_handler, we print "quit\n" when the 'cmd' variable
is EOF.  This happens when the incoming rl is nullptr.

This is what tries to print the 'quit' on the same line as the prompt, when the
user presses ctrl-d.

If we look at the readline manual, they have an example of using the callback
interface:

  https://tiswww.case.edu/php/chet/readline/readline.html#SEC43

In here, we see, in cb_linehandler, that when the incoming line is nullptr,
they explicitly print a `\n` character.  This isn't an arbitrary choice made
for this example, this is required, as, after seeing the ctrl-d readline emits
the BRACK_PASTE_FINI, which includes the '\r'.  Failing to move to the new line
will lead to any further output overwriting the prompt.

I'm currently trying to figure out if there's a better place where we can print
the 'quit' to work around this problem.

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