Multicast frames were being dropped by ieee80211_output_pbuf. It appears to look
up the destination address using cnx_node_search which only has an entry for the
broadcast address (all ones). This patch modifies cnx_node_search to return the
broadcast cnx_node for the multicast addresses too.
This is needed to support services such as mDNS on the softap interface.
Also adds source code for sdk_cnx_rc_search, adding a null pointer dereference
check (that is not expected to be seen), and source code for sdk_cnx_remove_rc.
Also code for sdk_cnx_sta_leave, although disabled due to accessing static date, but it shows a reference to the netif->flags and the NETIF_FLAG_DHCP flag removed in lwip v2.
