Updating submodule(s).

 [nomail]
Minor interface changes to provide more accessor methods for class
information.

In particular, adding a few const versions of methods.
A set of file analysis extensions.

- Enable manager to associate analyzers with a MIME type. With that,
  one can now say enable all analyzers for, e.g., "image/gif". This is
  exposed to script-land as

    Files::add_analyzers_for_mime_type(f: fa_file, mtype: string)

  For MIME types identified via libmagic, this happens automatically
  (via the file_new() handler in files/main.bro).

- Extend the analyzer API to better match that of protocol analyzers:

    - Adding unique analyzer IDs so that we can refer to instances
      from script-land.

    - Adding subtypes to Components so that a single analyzer
      implementation can support different types of analyzers
      internally.

    - Add an analyzer method SetTag() that allows to set the tag after
      construction.

    - Adding Init() and Done() methods for consistency with what other
      classes offer.

- Add debug logging to the file_analysis stream.

TODO: test cases missing for the new script-land functionality.
Minor API changes to analyzers.
Prettyfing Describe() for record types.

If a record type has a name and ODesc is set to short, we now print
the name instead of the full field list.
Checkpointing the dynamic plugin code.

This is essentially the code from the dynamic-plugin branch except for
some pieces that I have split out into separate, earlier commits.

I'm going to updatre things in this branch going forward.
Restructuring the plugin API to accomodate hooks.

I got rid of the earlier separate InterpreterPlugin class. Instead
Plugin now has a set of virtual methods HookSomething()... that
plugins can override. For efficiency purposes, they however need to
register first that they are interested in a hook, otherwise the
virtual method will never be called. The idea is to extend the set of
hooks over time as we figure out what's useful.

This is a checkpoint commit that's essentially untested and probably
broken. It compiles, though.
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.
Compile fix.
Updating submodule.
Removing a stale piece of CMake code.
Updating submodule.
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.