pf: unify DPFPRINTF definitions

We define DPFPRINTF() in a few different versions in most pf source files.
Move to one definition in pfvar.h and use that one everywhere.

While here change it to add an endline, as OpenBSD does, so we consistently end
all logs with an endline.
Also add a static probe point to ease future debugging. This way we can detect
log evens even when logging is disabled, and extract more information (e.g.
stack traces, frequency, ...).

Sponsored by: Rubicon Communications, LLC ("Netgate")