GPL wording fixups ("part of gdnsd")
plugin makefiles cleanup

Removed leading blank lines
Added -export-symbols-regex where missing
Re-ordered/formatted lists
symbol export stuff for libgdnsd
extmon: Add missing "static" keyword on some local constants
extfile: add missing CNAME support, update some warnings
Require IPv6 at runtime

We long ago dropped the configure-time argument to disable
compile-time IPv6 support.  This drops runtime support for
hosts with missing IPv6 support (usually due to a custom
kernel).  It's 2014; there's no good reason that people should
be hard-disabling IPv6 anymore on hosts that run authservers.

All code assumes working IPv6 at runtime.  The option
monitor_force_v6_up was also removed (it was a cheap workaround
for broken IPv6).  It's probably possible to make things work
anyways if you explicitly configure v4-only addresses, but if
that doesn't work I couldn't care less.
Misc docs/comments/warnings updates

Notably, all of the trivial plugins now have their own manpages.
plugin_weighted: remove 'cnames', update docs/tests

No functional change, just getting rid of a pointless
substanza that makes things more confusing.

Docs updates are for all the CNAME-monitoring-related
changes.