Noting test dependencies in our faq

Missing bit caught by sambuddhabasu...

  https://trac.torproject.org/projects/tor/ticket/13933
Gentoo doesn't have any connection resolvers

Well... damn. Looks like Gentoo doesn't have any connection resolvers we can
use...

  https://trac.torproject.org/projects/tor/ticket/13904#comment:17
Gentoo lies about proc permisssions

Gentoo says we have read permissions. Great! Only one minor hiccup, we get
permission denied if we read it...

  https://trac.torproject.org/projects/tor/ticket/13904#comment:19
Unit test failure for system_resolvers() on Gentoo

Oops, mistake in how platform detection is done. Making our Gentoo hacks play
well with the unit tests.
Revert removal of connection resolvers on Gentoo

Reverting our earlier change to drop connection resolvers due to tests failing
under Gentoo with grsecurity...

  https://trac.torproject.org/projects/tor/ticket/13904#comment:30