priv: don't output rc status twice when unable to open socket
lldpcli: add an option to enable promisc mode on managed interfaces

This allows LLDP frames to be received even when they are not in a
configured VLAN, like this may be the case with Cisco 2960.
lldpcli: give more details in the manual page about promiscuous mode
README: document the new promisc interface in README as well