Convert to the new thread safe gethostby{name,addr} nsswitch ABI.

branches: 1.3.6; 1.3.12;
Two new features:

- Detect and adapt to resolv.conf changes through the use of the new
res_check() API, so that for example changing the search list doesn't
require restarting clients.

- A persistent mdnsd connection pool, with slow start (so that
programs like ping, ssh, etc don't keep connections open) and age-out.
NOTE: Tuning parameter values are just SWAGs.

For the multicast_dns only mode, be very restrictive in what lookups we
try. No point in making queries and waiting for timeouts on lookups that
will never succeed.

Now only does lookups for linklocal addresses (v4 or v6) and
"name.local" names (and dotless names, iff "local" is specified in the
resolv.conf search list).

Also, lower the timeout in this mode, since multicast lookups should
return pretty fast or not at all.

Major enhancements to mdnsd nsswitch support:

- nss_mdns renamed to nss_multicast_dns for clarity and less chance of
confusion with nss_mdnsd (see next).
- Support using mdnsd for all unicast too with new nss_mdnsd module
(note: this mode requires an updated mdnsd too, not yet committed)
- Implement resolv.conf "search" directive support. Needed for nss_mdnsd,
potentially useful with nss_multicast_dns (you could now do
"search example.com local" in resolv.conf if you wanted to and it would
act as you might expect)
- Add references in nsswitch.conf man page and sample file
- Implement AI_CANONNAME
- Various bug fixes

Rebase to HEAD as of a few days ago.

sync with head.

for a reference, the tree before this commit was tagged
as yamt-pagecache-tag8.

this commit was splitted into small chunks to avoid
a limitation of cvs. ("Protocol error: too many arguments")