Improve userspace grant retrieval interface
(This is not necessarily kernel-specific, but because there are proc: APIs for this, I submitted it here.)
POSIX specifies that MAP_SHARED mappings shall remain in the forked child process, by reference, whereas MAP_PRIVATE mappings shall only remain by (at the time of copying) value (CoW). Since MAP_PRIVATE mappings may be managed individually by the scheme, relibc currently re-obtains grants using fmap, but the interfaces need to be improved.