More complex physalloc

A very basic implementation of syscall!46 (merged).

Adds a better interface for drivers when allocating physical memory, such as allocating in 32-bit physical memory space (required for some e.g. XHCI when a special capability register indicates that 64-bit memory is unsupported). Also adds "partial allocation", where a driver can request a number of frames, but only receive up to min, which is useful for scatter-gather lists.