- 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
the args for hash32_stre and hash32_strne but there are no consumers in the
base system and openbgpd does not use it which the initial import was for.
Silence on: hackers
