When pidptr was passed as NULL to pidfile_open(3), we were returning EAGAIN/EWOULDBLOCK when another daemon was running and had the pidfile open. We should return EEXIST in that case, fix it.
When pidptr was passed as NULL to pidfile_open(3), we were returning
EAGAIN/EWOULDBLOCK when another daemon was running and had the pidfile open.
We should return EEXIST in that case, fix it.

Reported by:	Dirk Engling <erdgeist@erdgeist.org>
Reviewed by:	jhb, Dirk Engling <erdgeist@erdgeist.org>
MFC after:	1 week

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase