nixos/hylafax: fix faxq `ModemGroup` setting

The manpage claims that the "limit" in the setting::
  <name>:[<limit>:]<regex>
is optional and defaults to zero, implying no limit.
However, tests confirmed that it actually isn't optional.

Without limit, the setting ``any:.*`` places
outbound jobs on infinite hold if no particular
modem was specified on the sendfax command line.
The new default value ``any:0:.*`` from
this commit uses any available modem to
send jobs if not modem was given to sendfax.