netpfil: Use accessor functions and named constants for all tcphdr flags

Update all remaining references to the struct tcphdr th_x2 field.
This completes the compatibilty of various aspects with AccECN
(TH_AE), after the internal ipfw "re-checksum required" was moved
to use the TH_RES1 flag.

No functional change.

Reviewed By: tuexen, #transport, glebius
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D43172

sys: Remove ancient SCCS tags.

Remove ancient SCCS tags from the tree, automated scripting, with two
minor fixup to keep things compiling. All the common forms in the tree
were removed with a perl script.

Sponsored by: Netflix

sys: Remove $FreeBSD$: one-line .h pattern

Remove /^\s*\*+\s*\$FreeBSD\$.*$\n/

IfAPI: Explicitly include <net/if_private.h> in netstack

Summary:
In preparation of making if_t completely opaque outside of the netstack,
explicitly include the header. <net/if_var.h> will stop including the
header in the future.

Sponsored by: Juniper Networks, Inc.
Reviewed by: glebius, melifaro
Differential Revision: https://reviews.freebsd.org/D38200

ipfilter: INLINE --> inline

Replace the INLINE macro with inline. Some ancient compilers supported
__inline__ instead of inline. The INLINE hack compensated for it.
Ancient compilers are history.

Reported by: glebius
MFC after: 1 month

ipfilter: Move kernel bits to netpfil

Through fixes and improvements our ipfilter sources have diverged
enough to warrant move from contrib into sys/netpil. Now that I'm
planning on implementing MSS clamping as in iptables it makes more
sense to move ipfilter to netpfil.

This is the first of three commits the ipfilter move.

Suggested by glebius on two occaions.

Suggested by and discussed with: glebius
Reviewed by: glebius, kp (for #network)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D33510