Improve guess support in rbt post.

This adds new abilities to rbt post's guess fields support. The old
guess support was a boolean state, which only supported opt-in and not
opt-out. That is, if GUESS_FIELDS was set to True in .reviewboardrc, it
couldn't be turned off when posting.

It's now a tri-state of "yes", "no", and "auto", which can be set on the
command line or .reviewboardrc.

If "yes", it behaves as before, with the fields being updated from the
commit message.

If "no", guessing will be turned off (which allows the command line to
override the config file).

If "auto", guessing will be turned on only for new review requests, but
turned off when updating.

If no --guess-* options or configuration variables are set, the
default will be "auto", making for a nicer experience when posting brand
new review requests.

If an old-style --guess-* option is passed without a value (in the form
of --guess-*=value or -gvalue), then the prior logic of forcing guessing
will be used. That is, "yes" is implied.

While "yes", "no", and "auto" are all valid values, we still support
True and False in .reviewboardrc, for backwards-compatibility.