Fix kill issue leading to zombie process on MacOS Sierra

Fix kill issue leading to zombie process on MacOS Sierra

Starting with MacOS version Sierra, the gdb kill command
seems to work but inferior remains as zombie on the host.
Notice that, as zombie process, the inferior is not killable
by the user, nor by root.

The kill signal gdb sent to the inferior is not handled
in gdb as a signal sent by gdb thus no reply is made and
the process remains (since MacOS does not "release" the
inferior because no reply have been made to the signal
message).

This patch fixes this problem.

gdb/ChangeLog
2018-08-02  Xavier Roirand  <roirand@adacore.com>

	PR gdb/22629:
        * darwin-nat.c (darwin_kill_inferior): Fix handling of
        kill inferior.