Disallow reject6 and accept6 lines in descriptors

Disallow reject6 and accept6 lines in descriptors

This fixes a remotely triggerable assert on directory authorities, who
don't handle descriptors with ipv6 contents well yet. We will want to
revert this once we're ready to handle ipv6.

Issue raised by lorth on #tor, who wasn't able to use Tor anymore.
Analyzed with help from Christian Fromme. Fix suggested by arma. Bugfix
on 0.2.1.3-alpha.