Add the missing variant in most places in the API where previously
there was either only a synchronous version or only an asynchronous
version.

There is not yet a synchronous nm_client_activate_connection(),
nm_client_add_and_activate_connection(), or
nm_remote_settings_add_connection(), because the existing async code
depends on waiting for other asynchronous events, so making them run
synchronously is slightly more complicated. But these APIs can be
added later.