Add a 'preupgrade' target which creates any missing users and groups

Add a 'preupgrade' target which creates any missing users and groups
(relative to FreeBSD-4) that are required before installworld can be
run.

Adjust the installworld prechecks to bail if required users or groups
do not exist, with a message pointing to the new preupgrade target.

The 'make upgrade' target will automatically run the preupgrade target.

Required users: smmsp, _pflogd, _ntp.
Required groups: smmsp, authpf, _pflogd, _ntp

Devon noted that the 'pw' program seems to do what we need here.