For /dev/mem, map in memory to be copied to memory's own address space

For /dev/mem, map in memory to be copied to memory's own address space
one page at a time, and use safecopies to copy it to the requesting
process.

This lets /dev/mem access the entire physical address space, as the minix
page tables only allow access by default to physical RAM, which breaks
e.g. the VESA X driver in some cases.