build: Rework ABI selection logic

build: Rework ABI selection logic

Add a new LIBBSD_SELECT_ABI m4 macro, and use it to setup the ABI
interfaces to expose and the various variables and conditionals
to be used by the build system.

Switch to set the initial values to unknown and then set every one
of the selections explicitly by supported target.

Update comments for rationale for things to DROP, or for why some
selections are enabled.