Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- 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

Fix a compiler warning so hash.h can be included in the kernel. This changes
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

Remove comment that does not appy to FreeBSD.

Import of OpenBSD's sys/sys/hash.h providing generic 32bit hash functions.

Requested by: flz (to port Open[BGP|OSPF]D)
MFC after: 3 days