GitHub Zeek/Bro Account Transition

The official location for Zeek (Bro) Git repositories is now here:

    https://github.com/zeek

The "bro" GitHub account (https://github.com/bro) is now only used to
mirror "zeek" repositories. Please use the later for new issues and
pull requests.

Many repositories with "bro" in their name have replaced that with
"zeek" at their new GitHub location. For example, the main "bro"
repository is now here:

    https://github.com/zeek/zeek

Also, the repositories hosted at git.bro.org are now just mirrors of
the GitHub account.

Existing git clones may continue to work, but here's an example of how
to update a clone of bro/zeek to the new URL (recommended):

    git remote set-url origin https://github.com/zeek/zeek
    git pull
    git submodule sync --recursive
    git submodule update --recursive --init

Note there is still quite a bit of renaming to be done within the
various codebases and documentation, but if you notice anything that's
outright broken as the renaming effort advances, feel free to report
it as it may have been overlooked.

- Jon