Don't allow -u on Mac OS X with pf proxyspecs

Apple checks EUID==0 on ioctl(/dev/pf), whereas OpenBSD and FreeBSD only
check permissions on open(/dev/pf).  This means that on OS X, it is not
possible to open /dev/pf, drop privileges, and send an ioctl to the file
descriptor opened earlier with EUID==0.  It also means Apple broke the
Unix way of dealing with device nodes - why are there file permissions
on /dev/pf when they later enforce EUID==0 on use, thereby breaking
basic Unix mechanisms?  Work around this by disallowing -u with pf
proxyspecs and by not automatically dropping to nobody on Mac OS X.

Issue:		#65
Reported by:	Vladimir Marteev