Remove fallback to fork(2) if pdfork(2) is not available. If the parent
process dies, the process descriptor will be closed and pdfork(2)ed child
will be killed, which is not the case when regular fork(2) is used.

The PROCDESC option is now part of the GENERIC kernel configuration, so we
can start depending on it.

Add UPDATING entry to inform that this option is now required and log
detailed instruction to syslog if pdfork(2) is not available:

The pdfork(2) system call is not available; recompile the kernel with options PROCDESC

Submitted by: Mariusz Zaborski <oshogbo@FreeBSD.org>
Sponsored by: Google Summer of Code 2013