Reworking plugin interface to not rely on macros.

The Plugin.cc file is now just a standard class, with the interface
changed a bit to make it more easy to write. However, there're still
some conventions that one must follow to make everything work (like
using the right namespace structure).

This commit also includes the option to compile built-in plugins
dynamically instead of statically by adding
SET(BRO_PLUGIN_BUILD_DYNAMIC TRUE) to their CMake config. This hasn't
been tested much yet, and I'm still undecided if it's somethign we
would want to do by default--but we could now if wanted. :)

Also some minor other cleanup of plugin APIs and built infrastructure.

All tested on MacOS only currently.
Moving existing built-in plugins over to new interface.
Updating submodules.