Clear signal mask early in main(); sshd may have been started with

Clear signal mask early in main(); sshd may have been started with
one or more signals masked (sigprocmask(2) is not cleared on fork/exec)
and this could interfere with various things, e.g. the login grace timer.

Execution environments that fail to clear the signal mask before running
sshd are clearly broken, but apparently they do exist.

Reported by Sreedhar Balasubramanian; ok dtucker@