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

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

ng_mppc(4):: basic readability cleanups.

In particular use __unreachable() to appease static analyzers.
No functional change.

CID: 1356591
MFC after: 3 days

ng_mppc(4): Bring netgraph(3) MPPC compression support.

Support for compression has been available from July 2007 but it
was never imported due to concerns with patents once held by
STAC/HiFn. The issues have clearly been resolved so bring it
in now.

Special thanks to Brett Glass for preserving the code and
pointing documentation for the expiration case.

Obtained from: mav (through Brett Glass)
Relnotes: yes
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D6739