This will make a number of things easier in the future, as well as (finally!)
avoiding the Id-smashing problem which has plagued developers for so long.
Boy, I'm glad we're not using sup anymore. This update would have been
insane otherwise.
traditional BSD4.4 behavior (_POSIX_SAVED_IDS are OFF) was described
before.
Add some hooks to easily change this text when
POSIX_SAVED_IDS model will be changed.