[PATCH 1 of 8 ipv6] runtests: export HGIPV6 to hint test scripts whether to use IPv6

[Danek Duvall]

Jun Wu wrote:

> With all the changes, it'll be easy to switch between v4 and v6 (by changing
> "useipv6" in run-tests.py). I think we can add a new command line flag to
> run-tests.py so people can choose whether they want to test v4 or v6.

Or both?  Would it be useful consider one to be primary and the other
secondary, and to always test only a subset of networking-related tests on
the secondary if it's available?  Running the entire testsuite twice just
to make sure that nothing's broken on both IPv4 and IPv6 seems like
overkill to me.

Danek