estop-latch: improve documentation; set default pin values
hm2-stepper: estop when hm2 watchdog bites

Any machine with a proper physical estop chain will need to revise
this setup, but for a basic setup it is better than the previous status
quo.  (Closes: #391)

Signed-off-by: Jeff Epler <jepler@unpythonic.net>
v2.6.3