Restore functionality for CookieAuthFileGroupReadable.

When we merged the cookieauthfile creation logic in 33c3e60a37, we
accidentally took out this feature.  Fixes bug 12864, bugfix on
0.2.5.1-alpha.

Also adds an ExtORPortCookieAuthFileGroupReadable, since there's no
reason not to.
Documentation fix on arguments to CookieAuthFileGroupReadable

We don't actually allow a group name, but the documentation implied
that we did.
Some documentation fixes for #12864.
Merge remote-tracking branch 'asn/nickm-bug12864_025' into maint-0.2.5