Infrastructure for modularizing protocol analyzers.

There's now a new directory "src/protocols/", and the plan is for each
protocol analyzer to eventually have its own subdirectory in there
that contains everything it defines (C++/pac/bif). The infrastructure
to make that happen is in place, and two analyzers have been
converted to the new model, HTTP and SSL; there's no further
HTTP/SSL-specific code anywhere else in the core anymore (I believe :-)

Further changes:

    - -N lists available plugins, -NN lists more details on what these
      plugins provide (analyzers, bif elements). (The latter does not
      work for analyzers that haven't been converted yet).

    - *.bif.bro files now go into scripts/base/bif/; and
      scripts/base/bif/plugins/ for bif files provided by plugins.

    - I've factored out the bifcl/binpac CMake magic from
      src/CMakeLists.txt to cmake/{BifCl,Binpac}

    - There's a new cmake/BroPlugin that contains magic to allow
      plugins to have a simple CMakeLists.txt. The hope is that
      eventually the same CMakeLists.txt can be used for compiling a
      plugin either statically or dynamically.

    - bifcl has a new option -c that changes the code it generates so
      that it can be used with a plugin.

TODOs:
    - "make install" is probably broken.
    - Broxygen is probably broken for plugin-defined events.
    - event groups are broken (do we want to keep them?)

scripts/base/frameworks/logging/main.bro

@@ -357,7 +357,7 @@ export {
 # We keep a script-level copy of all filters so that we can manipulate them.
 global filters: table[ID, string] of Filter;
 
-@load base/logging.bif # Needs Filter and Stream defined.
+@load base/bif/logging.bif # Needs Filter and Stream defined.
 
 module Log;