syslog(3): unbreak log generation using fabricated PID

Recover application ability to supply fabricated PID
embedded into ident that was lost when libc switched
to generation of RFC 5424 log messages, for example:

logger -t "ident[$$]" -p user.notice "test"

It is essential for long running scripts.
Also, this change unbreaks matching resulted entries
by ident in syslog.conf:

!ident
*.* /var/log/ident.log

Without the fix, the log (and matching) was broken:

Aug 1 07:36:58 hostname ident[123][86483]: test

Now it works as expected and worked before breakage:

Aug 1 07:39:40 hostname ident[123]: test

Differential: https://reviews.freebsd.org/D36005
MFC after: 2 weeks