Switch the listening select loop from select() to pselect() and

Switch the listening select loop from select() to pselect() and
mask signals while checking signal flags, umasking for pselect and
restoring afterwards. Also restore signals before sighup_restart
so they don't remain blocked after restart.

This prevents a race where a SIGTERM or SIGHUP can arrive between
checking the flag and calling select (eg if sshd is processing a
new connection) resulting in sshd not shutting down until the next
time it receives a new connection.  bz#2158, with & ok djm@