libnm contains the public function nm_utils_enum_from_str() et al.
The function is not flexible enough for nmcli's usecase. So, I would
need another public function like nm_utils_enum_from_str_full() that
has an extended API.
That was already required previously for ifcfg-rh writer, but in that
case I could just add it as internal API as libnm-core is linked statically
with NetworkManager.
I don't want to commit to a public API for an utility function. So move
the code instead to the shared directory, so that nmcli may link
statically against it and use the internal API.
