Added a placeholder for a plugin loading infrastructure.

This is based on a discussion between Dave MacMahon and Alec Rust
on the CASPER mailing list. A "load-plugin" command is added to the
raw katcp mode of tcpborphserver. This command will load a dynamic
library and add new commands to the server.

Currently it just prints the plugin name (hence place-holder).
Fleshed out the plugin instrasture.

Each plugin must contain an instance of struct PLUGIN (defined
in plugin.h) called KATCP_PLUGIN which itself contains an array
of structs PLUGIN_CMD which define new commands for katcp. The
dynamic loader looks for these and loads in commands appropriately.

Also, changed the load command to "?plugin-load" and added a
"?plugin-list" command which lists currently loaded plugins. There's
also "?plugin-unload" which should remove loaded plugins but this
remains to be written.
Wrote the unload plugin KATCP command.

Also made tracking the number of loaded plugins more robust.
It's still possible however to load a module more than twice, I
need to add a check to make that not allowable.