[branch-merge] Drop libpng requirements from base install

Right now, plymouthd links against libplybootsplash which links
against libpng.  Only graphical splashes use libpng, but it's
always pulled in.

This merge splits libplybootsplash into two libraries: the core
bits, and the extractable graphical bits.  Only the graphical splashes
link against the latter library.  This way server installations can
get plymouth without pulling in libpng.  This will reduce the amount
of security errata deployed to those server installations.