nwfilter: cleanup return codes in nwfilter subsystem

nwfilter: cleanup return codes in nwfilter subsystem

This patch cleans up return codes in the nwfilter subsystem.

Some functions in nwfilter_conf.c (validators and formatters) are
keeping their bool return for now and I am converting their return
code to true/false.

All other functions now have failure return codes of -1 and success
of 0.

[I searched for all occurences of ' 1;' and checked all 'if ' and
adapted where needed. After that I did a grep for 'NWFilter' in the source
tree.]