security/zeek: This adds security/zeek, the new version of security/bro.
This is being done as svn copy instead of rename so that users of
security/bro can have some time to migrate. It also allows for
possible security updates to the old bro port which upstream has
indicated is possible for at least a few months.

Reviewed by: ler (mentor)
Approved by: ler (mentor)
Differential Revision: https://reviews.freebsd.org/D22376