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

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

libkern: Fix two typos in source code comments

- s/noticably/noticeably/

MFC after: 3 days

s/ is is / is /g
s/ a a / a /g

Approved by: cperciva
MFC after: 3 days

- Move jenkins.h to jenkins_hash.c
- Provide missing function that can do hashing of arbitrary sized buffer.
- Refetch lookup3.c and do only minimal edits to it, so that diff between
our jenkins_hash.c and lookup3.c is minimal.
- Add declarations for jenkins_hash(), jenkins_hash32() to sys/hash.h.
- Document these functions in hash(9)

Obtained from: http://burtleburtle.net/bob/c/lookup3.c