reverse queries and dnsproxy

[Miika Komu]

When dnsproxy cache, hosts files or DNS fail with reverse resolution, we can
resort to Oleg's hit-to-ip extension in the dnsproxy.

Oleg's hit-to-ip extension currently works as follows:

Host A:

1. Enable "hit-to-ip on" in hipd_config
2. Enabled "nsupdate on" in hipd_config
3. Restart hipd

Host B:

1. Repeat steps of the of previous case
2. Make sure that the HIT of host A is not host files
3. ping6 HIT
4. hipd eventually executes hip_map_id_to_addr in order to find the HIT-IP
mapping from DNS
5. If found, hipd inserts mapping and triggers bex (otherwise hipd just
broadcasts to local lan)

Please write a wrapper to user.c for hip_map_id_to_addr and the corresponding
hipconf functionality. Then call it from dnsproxy to handle the reverse
mapping. Alternatively, you can call tools/nsupdate.pl script directly from the
command line.