configure: add --enable-7i80

currently no tests against Xenomai threads, so configure
with --with-xenomai
Makefile: hm2_eth module build
scripts/linuxcnc.in: rtnet start/stop

brute force. Will fail miserably if not xenomai, and not rtnet.
scripts/{start,stop}_rtnet.sh

absolute paths. Yikes.
hm2_eth: new files hm2_eth.{c,h},lpb16.{c,h}

applied cleanly
src/hal/drivers/mesa-hostmot2/hostmot2-lowlevel.h changes
src/hal/drivers/mesa-hostmot2/hostmot2.c changes
src/hal/drivers/mesa-hostmot2/tram.c changes
configure: add --with-rtnet=<rtnet install dir> option
xenomai-user: link hm2_eth.so (hopefully)