Add fs_modify_word to fieldset

You may modify an existing word to be a different value;
useful in UDP module for ICMP unreachable responses, so
we can update the "source" address
Move make_ip_str to probe_modules/packet.c

This is a utility function, mostly useful for probe modules
UDP probe module updates saddr fieldset

ICMP responses (e.g. DEST_UNREACH) to UDP probes (generally)
contain the IP/UDP packet that ellicited the response, though
they do not have to come from the host we sent them to (e.g.
NETWORK_UNREACH). We will "correct" this by switching the saddr
fieldset to be the host we WOULD have received a response from