Rearrange library code into separate files.

We split the library code into these separate files:

 - guestfs.c: creating handles, closing handles, handle-related variables
 - actions.c: generated library-side stubs for each action
 - bindtests.c: generated code to test bindings
 - launch.c: launching the appliance
 - proto.c: the library side of the daemon communications protocol

This is just code movement.

Cherry picked from commit 41f25ab3df5f306ac717fa7a6efd58328d30c1ae
and rebased for stable branch.