Add option to lockf to avoid creating a file if it does not exist.

PR:	bin/170775
Submitted by:	Matthew Story <matthewstory@gmail.com>
Reviewed by:	scottl
MFC after:	1 week
Permit relative paths with mergemaster -m

Reviewed by:	dim, brooks
MFC after:	3 days
Honor the net.inet6.ip6.v6only sysctl variable and the IPV6_V6ONLY
socket option for SCTP sockets in the same way as for UDP or TCP
sockets.

MFC after: 2 weeks