Add a header file to expose private utility functions from libnm-core
that can be used by NetworkManager (core) and libnm.so. The header
is also used to give privileged access to libnm-core. Since NM links
statically, these functions are not exported and not part of public ABI.

This also removes the NM_UTILS_PRIVATE_CALL() macro and libnm.so no
longer exports nm_utils_get_private().

Before, this functionality was partly declared in nm-utils-private.h.
This was wrong because nm-utils-private.h is for functionality
entirely private to libnm-core.

Signed-off-by: Thomas Haller <thaller@redhat.com>