configure.in:  ensure /bin and /usr/bin are in $PATH

Yes, /bin was actually missing from $PATH in a Wheezy debootstrap
chroot environment.
debian/control.in:  Build-Depends: kmod, procps

If ./configure doesn't find lsmod, ps and kill, the realtime and
linuxcnc scripts will break with empty $LSMOD, $PS and $KILL
variables.
runtests:  fix case where negative return code halts script

Replace the 'cmp' and 'diff' with just a single 'diff'.
FIXME:  temp. don't kill realtime.in to debug a msgd hang

Instead get a gdb back trace and then hang forever.