Replace SIGCHLD/notify_pipe kludge with pselect.

Replace SIGCHLD/notify_pipe kludge with pselect.

Previously sshd's SIGCHLD handler would wake up select() by writing a
byte to notify_pipe.  We can remove this by blocking SIGCHLD, checking
for child terminations then passing the original signal mask through
to pselect.  This ensures that the pselect will immediately wake up if
a child terminates between wait()ing on them and the pselect.

In -portable, for platforms that do not have pselect the kludge is still
there but is hidden behind a pselect interface.

Based on other changes for bz#2158, ok djm@