Allow PIC drivers to translate firmware sense codes for themselves. This
is designed to replace the tables in dev/fdt/fdt_ARCH.c, but will not
happen quite yet.
Do not map IRQs twice. This fixes PowerPC/FDT systems with multiple PICs,
which would try to treat the previously-mapped interrupts from
fdt_decode_intr() as interrupt line numbers on the same parent PIC.
Don't spin with mutex hold when there is not enough room in the send socket
buffer.  While here, make the code flow somewhat nicer.

Thanks to mav@ for tracking it down.

Tested by:	mav
MFC after:	3 days
Sponsored by:	FreeBSD Foundation