Fix maximum TTL info for consistency

It had been 2^24-1 earlier on, and some of the info still hadn't
been updated for the switch to 2^28-1 correctly.
Also, replaced sttl defines with static consts.
Switch to __thread TLS for dmn_log buffers

We had been using pthread_getspecific() because MacOS
didn't support TLS.  Apparently MacOS now supports TLS
(since about Lion).  I'm not terribly worried about
old Mac compatibility, as our Mac support is primarily
just for developer convenience and a minor help with
maintaining *BSD compatibility in general.
admin state user/tool input stuff

Basically, the file /var/lib/gdnsd/admin_state is watched with
an ev_stat watcher.  When it's present, it's parsed for vscf hash
entries like:
   127.0.0.1/www_svc => DOWN

The left-hand-side are actually glob patterns, and a TTL can optionally
be forced as well (subject to zonefile range clamping), e.g.
   127.0.0.1/* => UP/300

What's being matched on the left-hand-side is the "description", which
for true monitors is "addr_or_cname/service_type", and for virtuals is
plugin-defined (e.g. "geoip/res4/dc5").

An empty file is the same as one that's not present.  If an active file
is deleted, all forced states are cleared.