dahdi_genconf: Do not fail if pinned/assigned span utilities were not installed.

If you installed dahdi tools and did not specify DAHDI_PINNED=yes on the
makefile, when you run dahdi_genconf you would get an error like the following:

  # dahdi_genconf
  sh: span_types: command not found
  Command failed (status=32512): 'span_types dumpconfig > /etc/dahdi/span-types.conf' at ...

This change allows the generator for span-types.conf and span-assignements.conf
check for the existence of the utilities before attempting to call them.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>