Rewrite the main loop to use ppoll() instead of just blocking on read,
blocking the signals we care about when we aren't polling.

I didn't bother replacing alarm() with setitimer(); the alarm code
is dead anyway since there is no way for max_idletime to acquire a
non-zero value.

While here, avoid leaking the pid file and trigger descriptors to the
log child.

PR: 295840
MFC after: 1 week