Compare *reply_port to MACH_PORT_NULL, not implicit zero.

Compare *reply_port to MACH_PORT_NULL, not implicit zero.
(restore_gpr): Use N-1 as subscript into sc_gpr (sc_gpr[0] => $1).
Before general regs, restore from sc_mdlo and sc_mdhi.
Don't treat sp, fp specially; use restore_gpr for them too.
For final return, store user $1 value beyond top of user stack ahead of
time; Then use $1 to hold the user PC, and restore it from the stack in the
delay slot.