The next step for mimmutable(). ld.so figures out what regions of memory

The next step for mimmutable().  ld.so figures out what regions of memory
of startup shared library mappings can be made immutable, and also does
this for dlope() RTLD_NODELETE and subsidiary libraries.  Complexity in this
diff is due to the GNU_RELRO and OPENBSD_MUTABLE sections.
Tested in snaps for about 3 weeks, with some bootstrap related pain felt in ports
ok kettenis, much help from others.