Instead of using possibly complex ssh_signal(), write all the parts

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