Log Message:
Convert packages that test and use USE_INET6 to use the options framework
and to support the "inet6" option instead.

Remaining usage of USE_INET6 was solely for the benefit of the scripts
that generate the README.html files.  Replace:

	BUILD_DEFS+=	USE_INET6
with
	BUILD_DEFS+=	IPV6_READY

and teach the README-generation tools to look for that instead.

This nukes USE_INET6 from pkgsrc proper.  We leave a tiny bit of code
to continue to support USE_INET6 for pkgsrc-wip until it has been nuked
from there as well.