Add an option DB_FROM_SRC to use src/etc's user/group databases when
installing. This allows things like running installworld for 10-CURRENT
on a 9.0-RELEASE system without adding extra users and groups to the
passwd and group files.

To prevent potentially risky uid/gid mismatches on systems with
non-standard local values, require that DESTDIR be set if DB_FROM_SRC is
set.

Sponsored by: DARPA, AFRL
Reviewed by: peter