pfctl: use func rather than hardcoding function names for errors

Do for most running out of memory err() what was done for most running
out of memory log_warn(). i.e. ("%s", func) instead of manual
function names and redundant verbiage about which wrapper detected the
out of memory condition.

ok henning@

Obtained from: OpenBSD, krw <krw@openbsd.org>, a062aa9d6a
Sponsored by: Rubicon Communications, LLC ("Netgate")