ipfilter #ifdef cleanup.

ipfilter #ifdef cleanup.

Remove #ifdefs for ancient and irrelevant operating systems from
ipfilter.

When ipfilter was written the UNIX and UNIX-like systems in use
were diverse and plentiful. IRIX, Tru64 (OSF/1) don't exist any
more. OpenBSD removed ipfilter shortly after the first time the
ipfilter license terms changed in the early 2000's. ipfilter on AIX,
HP/UX, and Linux never really caught on. Removal of code for operating
systems that ipfilter will never run on again will simplify the code
making it easier to fix bugs, complete partially implemented features,
and extend ipfilter.

Unsupported previous version FreeBSD code and some older NetBSD code
has also been removed.

What remains is supported FreeBSD, NetBSD, and illumos. FreeBSD and
NetBSD have collaborated exchanging patches, while illumos has expressed
willingness to have their ipfilter updated to 5.1.2, provided their
zone-specific updates to their ipfilter are merged (which are of interest
to FreeBSD to allow control of ipfilters in jails from the global zone).

Reviewed by:	glebius@
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D19006