Safe userspace memory access mechanism

[losfair]

With the current UserPtr implementation we don't isolate userspace memory accesses from the rest of the kernel. The kernel dereferences user pointers directly, and VM faults from writing/reading a user address aren't handled.

Given that some platforms may provide special protection against userspace memory accesses (e.g. SMAP on x86), it would be nice to have copy_to_user/copy_from_user-like primitives in the HAL.