Add a layer of abstraction for the script bindings.

Add a layer of abstraction for the script bindings.
This layer of abstraction will allow to support any number of scripting
languages. Right now the support for scripting is built-in. However, it
won't be very difficult to exclude any built-in scripting support for a
particular language, and use dynamic modules instead.

If we decide that dynamic modules is the way to go, then apart from the
changes required for the build system, the only change in code will be in
script.c:LoadScripts(). There can be multiple modules each supporting a
particular scripting language, and the user will need to explicitly
specify which modules they want to use at start up (e.g., screen -M lua).
This will appease people who do want scripting support, but also people
who don't want it, and don't want the additional bloat.