New default for plugin installation path.
Cleanup of plugin component API.

- Move more functionality into base class.
- Remove cctors and assignment operators (weren't actually needed anymore)
- Switch from const char* to std::string.
Updates of the dynamic plugin code.

Includes:

    - Cleanup of the plugin API, in particular generally changing
      const char* to std::string

    - Renaming environment variable BRO_PLUGINS to BRO_PLUGIN_PATH,
      defaulting to <prefix>/lib/bro/plugins

    - Reworking how dynamic plugins are searched and activated. See
      doc/devel/plugins.rst for details.

    - New @load-plugin directive to explicitly activate a plugin

    - Support for Darwin. (Linux untested right now)

    - The init-plugin updates come with support for "make test", "make
      sdist", and "make bdist" (see how-to).

    - Test updates.

Notes: The new hook mechanism, which allows plugins to hook into Bro's
core a well-defined points, is still essentially untested.
Start of a plugin writing how-to.

See doc/devel/plugins.rst. It includes a simple example and background
on how things work.
Updating submodules.
Test updates.