Add -libs and -devel subpackages as well as incoporating
the changes that went into the Fedora package after its
package review.

The old approach of including the same source files into
multiple places is less than pretty.  But moving the code
to libply didn't seem right either, since libply is fairly
generic and could in theory get swapped out with a standard
runtime at some point.

Also, exposing libplybootsplash as an external library will
facilitate creating out of tree splash plugins.

Add:
    - fix error handling.  In particular, ply_open_module gets it completely
      wrong (replies on errno instead of dlerror())

It's just like boolean but doesn't advertise that it can
take the form --flag=false.  This makes the --help output
for --help say --help instead of --help={true|false}

For now, we still use the "--command" format.  Soon we'll
switch to the "command --option" format