pupnp: Log a warning if UPnP is already initialized

Currently, any flag other than UPNP_E_SUCCESS terminates UPnP during initialization. However, the UPNP_E_INIT flag (indicating UPnP is already initialized) should not cause termination. NAT-PMP has no equivalent flag.

Change-Id: Ia40508a4c07a141dbd5e2effe5f11dff05607d7c