upstream: Instead of using possibly complex ssh_signal(), write all

upstream: Instead of using possibly complex ssh_signal(), write all

the parts of the grace_alarm_handler() using the exact things allowed by the
signal-safe rules.  This is a good rule of thumb: Handlers should be written
to either set a global volatile sig_atomic_t inspected from outside, and/or
directly perform only safe operations listed in our sigaction(2) manual page.
ok djm markus

OpenBSD-Commit-ID: 14168ae8368aab76e4ed79e17a667cb46f404ecd