realtime:  thorough shmdrv checks; DEBUG value always decimal

Unload() was leaving the shmdrv module behind when segments were
allocated, even if not open.

FIXME:  this case needs to be clarified; see
    https://github.com/zultron/linuxcnc/issues/36

CheckShm() now takes an argument for which specific shmdrv check:
     -l: return success if shmdrv device exists
     -w: wait for shmdrv to be loaded; return success if loaded
     -a: return success if any segments allocated
     -o: return success if any segments open

Also, always set $DEBUG to some decimal number.