Add vm_extra_pre_umount() prototype to vmimage.subr.

Sponsored by:	The FreeBSD Foundation
Fix DESTDIR for installworld, and make sure it is created
before use.

Sponsored by:	The FreeBSD Foundation
Move usage() from vmimage.subr to mk-vmimage.sh,
in case vmimage.subr has not been sourced.

Sponsored by:	The FreeBSD Foundation
Spell 'OPTARG' correctly.
Actually call vm_create_base().

Sponsored by:	The FreeBSD Foundation