Most of these changes are either cmake-related or plugin-related.
Added a new test "plugins/legacy.zeek" to test that legacy Bro plugins
still work.
Also added a symlink bro-path-dev.in because some legacy Bro packages
won't install without it.
This also installs symlinks from "zeek" and "bro-config" to a wrapper
script that prints a deprecation warning.
The btests pass, but this is still WIP. broctl renaming is still
missing.
#239