Add SUBDIR_ORDERED, a generic mechanism for ordering subdirectories
This restores the sorting removed in r320701, broken by the SUBDIR.yes
work, and applies it to SUBDIR + SUBDIR.yes, providing equivalent
functionality to the sorting before SUBDIR.yes was added.
The uniquifying piece (:u) doesn't seem like it's really useful in my
opinion, so I didn't readd that feature.
Documentation in bsd.README pending.
MFC after: 2 months