This is a first cut of a plugins implementation for Bacula.
The current code is simply a test program named main, which
when executed will load the plugin named plugin.so and then
call it with a few calls, then exit.

What is implemented:
- load a plugin
- get main entry points in plugin (two)
- pass plugin Bacula entry points
- get plugin's entry points
- keep list of plugins that are loaded 
- The plugin currently only needs config.h and bc_types.h
  in the Bacula src directory to be build.  Otherwise, it is
  totally independent of Bacula.
- The main test program is intended to be integrated into
  Bacula (at least a number of the functions therein).

What is not yet implemented:
- Search for all plugins in the plugin directory
- Implement Bacula context for plugin instances
- Implement plugin context for plugin instances
- Some better method to pass variables
- Pass Version information in plugin Initialize
- Define more functionality (Bacula entry points, ...)
- Integrate the code into Bacula
- Make libbac.a into a shared object and allow plugin
  to call functions in directly so that plugins can use
  the same "safe" system utilities that Bacula uses.
- Document the interface
- Document libbac.so (when implemented)
- Write a couple of plugins that really do something.
- Figure out a way to deal with 3 types of plugins (Director,
  File daemon, Storage daemon).
- Error handling must be much improved and brought into Bacula
  programming style. It currently just printf()s a message then
  exits.