hurd: Fix restoring reply port in sigreturn

We must not use the user's reply port (scp->sc_reply_port) for any of
our own RPCs, otherwise various things break. So, use MACH_PORT_DEAD as
a reply port when destroying our reply port, and make sure to do this
after _hurd_sigstate_unlock (), which may do a gsync_wake () RPC.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>