Some (backwards-compatible) changes in mmap() call message fields
that allow for a 64-bit offset. minix_mmap() takes an off_t and
minix_mmap64() takes a u64_t. Some mmap() work in VM goes into a
separate function, using the new fields, so that that can be re-used
when files are to be mapped (future commit).
Change-Id: Ifb77a90b593dd3c33cf81b396068e4da1ec5fb1c