FreeTDM: Add trunk mode (NET/CPE) to span configuration in freetdm.conf and use it in ftmod_misdn and ftmod_libpri.

Some I/O implementations (e.g. mISDN) need this information to correctly
set up the port configuration. In FreeTDM, the mode of a span has been.
up until now, a part of the signalling module configuration ("node" or "mode"
for libpri and isdn spans), which is parsed _after_ the I/O part of a span
has been initialized. This limitation currently prevents us from using mISDN
in NT mode.

To work around this problem (without adding a lot of new tunk_type values like "E1_NET" etc.),
add a new "trunk_mode" parameter to the span categories in freetdm.conf, which by default
is "CPE" for all trunk types, except "FXS" (which defaults to "NET").

ftmod_misdn uses trunk_type + trunk_mode to correctly choose the d-channel protocol
for a port.

ftmod_libpri uses trunk_mode as a hint for the default "mode" setting.
(NOTE: It will print a warning if trunk_mode and signalling mode do not match.)

All other modules currently ignore the value of trunk_mode.

Example freetdm.conf snippet for a mISDN span in NET/NT mode
(using a HFC-S USB dongle):

  [span misdn MBRI_1]
  trunk_type => BRI_PTMP
  trunk_mode => NET
  b-channel  => HFC-S_USB.1:1-2
  d-channel  => HFC-S_USB.1:3
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>