FreeTDM: Add gcc printf()-style format string checks to ftdm_log(), also add FTDM_(U)INT64_FMT and FTDM_TIME_FMT constants.

The format string checks already caught a couple crash-worthy bugs and this
commit fixes a couple more.

Also includes __ftdm_check_scanf(), for completeness (currently unused).
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>