Ensure all standard targets honor SUBDIR. Now `make obj' descends into
SUBDIRs even if NOOBJ is set (some descendants may still need an object
directory, but we do not have such precedents). Now `make install' in
non-bsd.subdir.mk makefiles runs `afterinstall' target _after_ `install'
in SUBDIRs, like we do in bsd.subdir.mk. Nothing depended on the wrong
order anyway.
Fixed `distribute' targets (except for the bsd.subdir.mk version) so that
they do not depend on _SUBDIR; `distribute' calls `install' which already
depends on _SUBDIR.
De-standardize `maninstall', otherwise manpages would be installed twice.
(To be revised later.)