nat: Deprecate selector hostname, fix value to ""

nat: Deprecate selector hostname, fix value to ""

Its value has always been "", unless the deity recompiled with
RESOLVE_IPADDRESS defined, or imported another value with empdump.

I'm going to drop the RESOLVE_IPADDRESS code, including struct natstr
member nat_hostname.  To prepare for that, fix the selector's value to
"", so it doesn't need nat_hostname, and deprecate it.

This changes its "len" in xdump meta nat from 512 to 0.  Unlikely to
upset clients.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>