simplify plugin.h macros
Refactor logf_dname() into gdnsd_dname_to_string(), etc.

logf_dname() was the only dname-to-string formatter available, and
its output was by definition in the log format buffer, making it
unsuitable for general use.  This commit:

(a) Moves the core formatting function to
    gdnsd_dname_to_string() for general use outside of a log
    formatter, which logf_dname() is now a thin wrapper around
    for logging purposes.
(b) Refactors and simplifies the formatting code itself.
(c) Gets rid of the case where zscan_djb was using logf_dname()
    outside of a logging context for lack of a better option.
mark dmn logger functions as cold

Marking the logger functions as cold hints to gcc that branches
leading to them are unlikely().  By design we don't spam log
output.  Because log output is rare and almost always associated
with some kind of error or anomaly, it's by definition unlikely.

This patch also removes (now-redundant) explicit unlikely()
wrappers sprinkled all over the code which lead unconditionally to
log output.
ARRAY_SIZE macro
Support ?f=1 query arg for stats flush-after-fetch deltas

The stats URLs now support a query parameter "f=1" to request
stats flush-after-fetch behavior.  Stats queries that specify f=1
get counter values since the previous f=1 request rather than
since daemon start (unless it's the first such request since
daemon start, obviously).  Stats queries that do not specify the
f=1 argument are unaffected and continue to see the full counts.

Note that the f=1 state is global: if two independent clients are
querying with f=1, they'll reset each others' view of the stats!

Also note that rollover issues in the underlying counts are going
to screw this up pretty bad on 32-bit machines with large amounts
of traffic (but that was true even without this feature.  Use a
64-bit machine if you care!)

This updates the HTTP "parser" a bit as well.  Ultimately I still
think the mini-pseudo-HTTP server should die, but this is a decent
and simple feature to tide some over until it's replaced with
something better.

Issue #76