logging: Persistent children include pid in their log messages

Use the new PHASE_CHILDPERSIST hook to discover when we fork and
become a persistent child.  When that happens, we call openlog() again
to add LOG_PID, and we make a note to add the pid to messages written
directly to logfiles (or the terminal) by secnet's file logger.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>