ftmod_libpri: Add configure check for pri_maintenance_service()

Said function has been added in libpri-1.4.11 and breaks
building with older libpri versions (i.e. the passive one for ftmod_pritap).

Check for pri_maintenance_service() in configure and disable
the "maintenance" subcommand if it is not available.

Signed-off-by: Stefan Knoblich <stkn@openisdn.net>
ftmod_libpri: Also conditionally disable pri_set_service_message_support() call

Same as pri_maintenance_service(), use HAVE_LIBPRI_MAINT_SERVICE.

Signed-off-by: Stefan Knoblich <stkn@openisdn.net>
ftmod_libpri: Disable on_facility() event handler on older libpri versions

Those that lack PRI_EVENT_FACILITY (<= 1.4.10)

Signed-off-by: Stefan Knoblich <stkn@openisdn.net>