MFC:
r241684,245316,245318,245440,245563,245565,245622,245754,246092,246096,246602

r241684:
Allow LOCAL_DIRS to install files in directories not found in the
system mtree files via a LOCAL_MTREE variable which contains a list of
mtree files to be applyed along with the base mtree files to the tmp
root and DESTDIR.

r245440:
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

r245565:
Rework the mtree portion of etc/Makefile's distrib-dirs target to run
mtree in a shell loop so there is only one mtree commandline.  Move the
implementation of LOCAL_MTREE into etc/Makefile.

Sponsored by:   DARPA, AFRL
MFC r245265,245271,245309:

Rather than using zic to both compile and install zoneinfo files,
generate the files during the build and install them with install(1).
This was the one place in installworld where files (vs links) were
installed by a tool other than install.

Reviewed by:	edwin, jilles