Undo soname bump

Undo soname bump

The soname was bumped before 3.2.18 was released to reflect
an ABI change in the cache and object API.

This caused a discussion with various distribution maintainers
involved as it prevented them from including the release in
their stable branches. This is especially unfortunate as the
corresponding API is unused outside of libnl as of now.

For this reason, I have decided to undo the soname bump and
instead remove the corresponding header files from the list of
installable headers and thus remove them from the public API.

This is a API break technically but since nobody is using
them it should not matter. If you are in fact using any of the
*-api.h headers please let me know and we are most likely to
start a new development branch.

If your application linked against 3.2.18 or 3.2.19 you will need
to relink if you want it to use >= 3.2.20.

Signed-off-by: Thomas Graf <tgraf@suug.ch>