Plugins that were made using an older skeleton (e.g. from Bro 2.5.x)
may still reference "bro-config.h", "bro-path-dev", or "bro" from the
build tree, so we can simply provide compatibility symlinks/wrappers
for the new files that use "zeek" in their names.
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
