PR gdb/20604 - fix "quit" when an invalid expression is used

PR gdb/20604 - fix "quit" when an invalid expression is used

This fixes PR gdb/20604.  The bug here is that passing an invalid
expression to "quit" -- e.g., "quit()" -- causes gdb to enter a
non-functioning state.

The immediate problem is that quit_force resets the terminal before
evaluating the expression.  However, it seemed to me that it doesn't
really make sense to pass the quit_force argument to kill_or_detach
(which passes it to to_detach), first because conflating the exit
status for "quit" and the signal to pass when detaching doesn't make
sense, and second because to_detach implementations generally only
accept a constant here, while "quit" accepts an expression.  So, I
removed that.

As an aside, I think the "detach SIGNO" functionality is not
documented.

Built and regtested on x86-64 Fedora 24.

2016-09-21  Tom Tromey  <tom@tromey.com>

	PR gdb/20604:
	* top.h (quit_force): Update.
	* top.c (quit_force): Changed type of first argument.  Don't
	evaluate expression.  Pass NULL to kill_or_detach.
	* cli/cli-cmds.c (quit_command): Evaluate "args".

2016-09-21  Tom Tromey  <tom@tromey.com>

	PR gdb/20604:
	* gdb.base/quit.exp: New file.