Currently we don't accept writable file mmap()s, as there is no
system in place to guarantee dirty buffers would make it back to
disk. But we can actually accept MAP_SHARED for PROT_READ mappings,
meaning the ranges aren't writable at all (and no private copy is
made as with MAP_PRIVATE), as it turns out a fairly large class of
usage.
. fail writable MAP_SHARED mappings at runtime
. reduces some minix-specific patches
. lets binutils gold build on minix without further patching
Change-Id: If2896c0a555328ac5b324afa706063fc6d86519e
Minix-specific changes were needed in msg_sys.def:
* use minix_mmap()/minix_munmap() since mmap()/munmap() aren't
defined on Minix.
* use MAP_PRIVATE flag since MAP_SHARED isn't defined on Minix.
Tested with usr.bin/menuc/testm.
Change-Id: I1c463e7b0623036050adb6a9079c6747e1c1e084