Stop using _SUBDIR internally for non-SUBDIR_PARALLEL builds.

This is unifying more of the logic. Rather than create targets such
as 'all: all_subdir_foo' when using SUBDIR_PARALLEL and using
'all: _SUBDIR' when not using SUBDIR_PARALLEL, always use the
expanded out <target>_subdir_<directory> pattern. When not using
SUBDIR_PARALLEL, have each directory-target depend on the previously
defined targets as to respect the *order* of SUBDIR.

Using 'make -N' now prints all directory traversals individually rather
than using a loop, since a loop is no longer used to traverse.

This is part of an effort to cleanup handling of some edge cases
involving 'make <directory>' and making it simpler in the sense
that the pattern used to build is the same for all modes.

MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division