FreeTDM: Rewrite "ftdm core" print_* functions (to use new span iterator)

Simplify code by using shared string constants using bool (0/1) index,
!!-operator and negation-via-XOR.

Future optimization: Preallocate and re-use of iterators, to avoid
repeatedly allocating and freeing of memory.
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>