Don't bother trying the reverse-mapping through ifAlias trick, since
ifAlias is of course non-unique, if the user configures the same
description for multiple ports (e.g., "open port").
The LLDP spec says that if lldpRemPortIdSubtype is
"interface name", then lldpRemPortId should contain
ifName. It doesn't say much about lldpRemPortDesc;
some vendors seem to always leave it empty, some use
the interface description / ifAlias. Our algorithm
for lldp_port is now basically: If we find that
lldpRemPortIdSubtype is "interface name", then use
the lldpRemPortId, else use lldpRemPortDesc if set,
else use lldpRemPortId.
Bill Fenner