func(short) and the fact the the kernel uses full ints. This caused
problems on the pc532 port. These fixes take the good 16 bits passed
by the user program and converts them into the correct form for the
kernel.
fix jolitz's trapsignal() change (it's only supposed to work on curproc)
and add a panic if called w/something other than curproc, and also
fix a relic of his crappy exec implementation