Rework r289778 to always parallelize known targets, without ordering.

• Rather than allow 'make clean*' to ignore dependencies, make a static list of targets in STANDALONE_SUBDIR_TARGETS that are known to be safe. This allows a user to override them if needed and avoids adding this feature to user-defined targets that are in ${SUBDIR_TARGETS}. [1]
• This now also allows to force SUBDIR_PARALLEL when calling these targets, since no dependencies are needed.

Reported by: ian [1]
Sponsored by: EMC / Isilon Storage Division
MFC after: 3 weeks
X-MFC-With: r289778