Add stricter checking of some mmap() arguments:

• Fail with EINVAL if an invalid protection mask is passed to mmap().
• Fail with EINVAL if an unknown flag is passed to mmap().
• Fail with EINVAL if both MAP_PRIVATE and MAP_SHARED are passed to mmap().
• Require one of either MAP_PRIVATE or MAP_SHARED for non-anonymous mappings.

• Just trying to boot with the original patch forced me to relax the requirement for at least one of MAP_SHARED or MAP_PRIVATE to not be applied for anonymous mappings because libthr uses MAP_ANON without a sharing mode.
• I wrote a test program to exercise these various new errors as well as several normal use cases. All of the tests succeed on current HEAD, including the ones that pass invalid flags or protection that this change checks for. With the patch, the latter tests now fail.