Make simpledrm selection configurable from config file

Make simpledrm selection configurable from config file

Move the handling of the "plymouth.use-simpledrm" commandline argument from
ply-device-manager to main.c, and allow specifying its default value in
the "[Daemon]" section of the config-file using a new UseSimpledrm keyword.

Extend the "plymouth.use-simpledrm" handling to also accept a value e.g.
"plymouth.use-simpledrm=0" to allow overriding a "UseSimpledrm=1" in
the configfile.

And pass the use-simpledrm value to ply_device_manager_new () through a new
PLY_DEVICE_MANAGER_FLAGS_USE_SIMPLEDRM flag.

This also moves the kernel commandline handling to src/main.c grouping
it together with most of the other commandline handling.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>