library, not header.  Eliminate need for <ac/unistd.h> to sometimes
include <fcntl.h> and/or <sys/file.h>.  Change lock API to expect
fd not FILE*.  Allows wider use and eliminates requirement that
lutil_lockf.h depencency on stdio.h.
Implemented lockf, fcntl, and flock locking in lutil/lockf.c.
Additional implementations (including no-op) may be needed.
Update slapd/lock.c and slurpd/lock.c to use new API.