Stem errors when it sees reject6 or accept6 exit policies

Tor has a couple exit policy keywords (reject6 and accept6) which break just about any bit of Stem that reads exit policies.

10:55 < toralf> atagar: "ExitPolicy reject6 *:*" raises in
controller.get_exit_policy() : ValueError: An exit policy should have
a space separating its accept/reject from the exit pattern: reject6
*:*

11:07 < atagar> toralf: Interesting! I keep an eye on changes to the
control and directory specifications but this didn't include a change
to either so 'reject6' is a surprise for me. Joy, guess this'll
warrant a hotfix for the recent 1.4.0 release.

11:07 < atagar> Thanks for the heads up. :P

Definitely something we should fix. Nyx evidently had a longstanding issue around this too (#10579 (closed)). Not an overly big whoop though. First, they're extremely rare and second, unless I'm missing something they're pointless. Exit policy addresses fall into three camps...

• wildcard (for instance 'accept *:80')
• ipv4 (for instance 'accept 0.0.0.0/8:80')
• ipv6 (for instance 'accept [0000:0000:0000:0000:0000:0000:0000:0000]/64:80')

The first camp (wildcards) match against ipv4 or ipv6. The later two are for ipv4-only or ipv6-only policy rules. Or at least that's how Stem treats it.

Unless I'm missing something reject6 is just an alias for... what? The man page entry added as part of #12878 (moved) is the only description it seems to have ever gotten, and personally can't say I'm finding it too illuminating.