GDBserver: ctrl-c after leader has exited

GDBserver: ctrl-c after leader has exited

The target->request_interrupt callback implements the handling for
ctrl-c.  User types ctrl-c in GDB, GDB sends a \003 to the remote
target, and the remote targets stops the program with a SIGINT, just
like if the user typed ctrl-c in GDBserver's terminal.

The trouble is that using kill_lwp(signal_pid, SIGINT) sends the
SIGINT directly to the program's main thread.  If that thread has
exited already, then that kill won't do anything.

Instead, send the SIGINT to the process group, just like GDB
does (see inf-ptrace.c:inf_ptrace_stop).

gdb.threads/leader-exit.exp is extended to cover the scenario.  It
fails against GDBserver before the patch.

Tested on x86_64 Fedora 20, native and GDBserver.

gdb/gdbserver/
2014-11-12  Pedro Alves  <palves@redhat.com>

	* linux-low.c (linux_request_interrupt): Always send a SIGINT to
	the process group instead of to a specific LWP.

gdb/testsuite/
2014-11-12  Pedro Alves  <palves@redhat.com>

	* gdb.threads/leader-exit.exp: Test sending ctrl-c works after the
	leader has exited.