build: merge branch 'symbol-versioning'

No longer export all symbols of the shared libraries
but pick them explicitly. Hide symbols that were
private and not part of a public header file.

This is an ABI change, but hopefully not too painful.

There are still symbols in the header files that should
be private. They continue to be exposed.

http://lists.infradead.org/pipermail/libnl/2015-January/001794.html