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.

Reviewed by: alc, kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D698