Use the offsets from pcb.h rather than regnum.h to store the registers
in the pcb. setjmp/longjmp in the kernel also used these values, so
continue to use them although their use isn't technically the pcb
register array (matching is all that's important for setjmp/longjmp in
the kernel). Finally, eliminate the old register names from regnum.h.

This is a lexical change only. The non-debug .o files have the same md5.
Minor whitespace nit
Fix couple of mbuf leaks in incoming ARP processing.