We need to make sure that the worst thing that a weird consensus param
can do to us is to break our Tor (and only if the other Tors are
reliably broken in the same way) so that the majority of directory
authorities can't pull any attacks that are worse than the DoS that
they can trigger by simply shutting down.
One of these worse things was the cbtnummodes parameter, which could
lead to heap corruption on some systems if the value was sufficiently
large.
This commit fixes this particular issue and also introduces sanity
checking for all consensus parameters.
There are now four ways that CBT can be disabled:
1. Network-wide, with the cbtdisabled consensus param.
2. Via config, with "LearnCircuitBuildTimeout 0"
3. Via config, with "AuthoritativeDirectory 1"
4. Via a state file write failure.
This just got a little complicated, since old clients use "clipped
advertised bandwith" and new clients now use "consensus bandwidth" but
fall back to "clipped advertised bandwidth".
Fix several bugs in computing recommended versions. 1) refactor is-this-version-good handling and which-vesions-are-good handling to be in the same place. 2) a version is recommended if more than half of the versioning authorities like it, not >= half. 3) "NEW_IN_SERIES" should mean, "I don't know of an 0.1.1.x this recent, and there are some 0.1.2.x versions out", not "I don't know of an 0.1.1.x this recent, but I know some older ones." This should resolve bug 383.
svn:r9523
Write the entry guards section of path-spec; note a possible bug in cirbuitbuild.c; add a const; defer work on torrc.complete to be part of a bigger config documentation reorg.
svn:r9465
