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.

This will be useful for changing the client to use a

plymouth subcommand --option1 --option2

format

This is just a thin wrapper around the buffer object,
that keeps the data in terms of "elements" instead of
bytes, and always null terminates the result.

Copy and paste ply-logger into a more general ply-buffer class,
so we have a facility to buffer key strokes and process them as necessary