natpmpc: set lifetime via -l option, see #6
natpmpc: refactor argument parsing, see #6

The refactored argument parsing lets the user define
multiple port mappings in one go.
natpmpc: adds -v (verbose) flag, see #6

also: write debug output to stderr, not to stdout.
natpmpc: stricter error checking, see #6
natpmpc: fixed argument parsing, see #6
Merge branch 'nat-pmp'