Remove indirection of _sub target for using _SUBDIR.

This reverts r266473 as the need for it, working around .MAKE and '+'
issues, is no longer needed after r289460. This avoids extra log
output in -j builds of '-- _sub.TARGET --' that are redundant with the
'-- TARGET --' and '-- TARGET_subdir_DIR --' entries already showing.

r266473 also made a subtle change in the ordering of _SUBDIR handling. Before
the change, SUBDIRS were recursed into after building the TARGET due to the
.USE of _SUBDIR *appending* the commands onto the TARGET. After the change
though the indirection caused TARGET to depend on _sub.TARGET which had the
_SUBDIR handling in it. This TARGET would run after recursing. However, the
SUBDIR_PARALLEL handling from r263778 has this ordering as well. Since
this has so far not been a problem, for now make this behavior for
non-SUBDIR_PARALLEL use of _SUBDIR explicit by using .USEBEFORE.
Further research may change this back to .USE as well as the
SUBDIR_PARALLEL handling and bsd.progs.mk recursing.

Sponsored by: EMC / Isilon Storage Division