build: Make almost all exposed interfaces use the new ABI selection

build: Make almost all exposed interfaces use the new ABI selection

Explicitly select what to include as part of the target ABI, instead of
letting autoconfiguration potentially break ABI if the system grows
functionality provided by the library.

Make almost all the library selectable per target. Do not install manual
pages for interfaces not included in the library. Control inclusion of
symbols in map file via pre-processor macros, and move the comments
describing the ABI selection to configure.ac.

For now the header files are included as is and filtered through
pre-processor conditionals. Eventually they might get switched to be
autogenerated at build time.