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.

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

ply_command_parser_get_help_string and
ply_command_parser_get_options never got added

This will be useful for changing the client to use a

plymouth subcommand --option1 --option2

format