The call to mtree should not be necessary as the directories should already
have been created.
This removes thousands of lines of warning output from make installworld
with -DNO_ROOT:
.: user (0, 3375, not modified: Operation not permitted)
gid (0, 3375, not modified: Operation not permitted)
share: user (0, 3375, not modified: Operation not permitted)
gid (0, 3375, not modified: Operation not permitted)
share/misc:
user (0, 3375, not modified: Operation not permitted) gid (0, 3375, not modified: Operation not permitted)
etc...