settings: introduce abstract settings

Settings will be used to express configurable parameters in the
rendering modules and fb backends.

The goal is to address both commandline argument setting of parameters,
automatic use of defaults, as well as interactive configuration
including the outputting of the resulting settings in a form usable as
a commandline for future reuse.

Since settings can be numerous and highly varied from one module or
backend to another, a form similar to the Linux kernel's cmdline or
QEMU's approach has been adopted.

For example, a complete DRM backend, card selection and config would be:
rototiller --video=drm,dev=/dev/dri/card0,connector=LVDS-1,mode=1024x768@60

If any of the above were omitted, then the missing settings would be
interactively configured.

If you added --defaults, then any omissions would be automatically
filled in with the defaults.

i.e.
rototiller --video=drm,dev=/dev/dri/card4 --defaults

would use the preferred connector and mode for that card.

rototiller --video=drm --defaults

would do the same but also default to the /dev/dri/card0 path.

for brevity, I omitted rendering modules from above, but the same
approach applies simply to --module=:

rototiller --module=sparkler --video=drm --defaults

If you ran rototiller without any arguments, then a fully interactive
setup would ensue for module and video.

If you ran rototiller with just --defaults, then everything is
defaulted for you.  A default rendering module will be used (the
original roto renderer, probably).

Note that this commit only adds scaffolding to make this possible,
none of this is wired up yet.

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