[BUG] gdb: quit hangs after step into signal handler

[Oleg Nesterov <oleg@redhat.com>]

In short, gdb uses ptrace(PTRACE_KILL) + wait() to detach/kill the tracee.
Well, I think PTRACE_KILL should be avoided but this is offtopic,
ptrace(PTRACE_CONT, SIGKILL) has the same problem if the tracee has
stepped into the signal handler.

	$ gdb /usr/bin/perl
	GNU gdb (GDB) 7.1
	...
	(gdb)
	(gdb) set args -e '$SIG{HUP}=sub{}; kill HUP, $$; 1 while 1'

(this sets a signal handler for SIGHUP, sends SIGHUP to itself, then
 spins forever)

	(gdb) r
	Starting program: /usr/bin/perl -e '$SIG{HUP}=sub{}; kill HUP, $$; 1 while 1'
	[Thread debugging using libthread_db enabled]

	Program received signal SIGHUP, Hangup.
	0x000000375fa32507 in kill () from /lib64/libc.so.6
	(gdb) si
	0x00000037522a2a40 in Perl_csighandler () from /usr/lib64/perl5/5.10.0/x86_64-linux-thread-multi/CORE/libperl.so

(step into handler)

	(gdb) q
	A debugging session is active.

		Inferior 1 [process 9694] will be killed.

	Quit anyway? (y or n) y

(ptrace(PTRACE_KILL) + wait)

Hangs "forever" in wait4().

At first I thought this is another bug in ptrace-utrace (and I even
started doing the patch), but then I recalled that this is what
upstream does, and ptrace-utrace just tries to be compatible.

There is no signal context after step-into-handler, ptrace(PTRACE_KILL)
or ptrace(PTRACE_CONT, SIGKILL) acts like ptrace(CONT, pid, 0,0) and
just resumes the tracee.

Probably gdb should just do kill(tracee, SIGKILL) when it wants
to terminate the tracee.

Oleg.