Add generic driver for "NEC PC-9801(*) extension board slot" on

Add generic driver for "NEC PC-9801(*) extension board slot" on
LUNA-88K.

LUNA-88K{,2} has one or two slot(s) that can attach the extension
board designed for PC-9801.  This driver provides dedicated mmap(2)
and capability for waiting specified interrupt on that slot so that we
can use the extension board from userland program.

(*)PC-9801 is a Japanese popular personal computer, mainly used in
   1980-90's. (see http://en.wikipedia.org/wiki/NEC_PC-9801)

ok miod@, and man pages jmc@