snmp: implement lldpRemOrgDefInfoTable for remote custom TLVs

snmp: implement lldpRemOrgDefInfoTable for remote custom TLVs

As a simplification, lldpRemOrgDefInfoIndex is 1 for the first custom
TLV of a given port and is increased by 1 for each new TLV. This is
not what is encouraged in the MIB:

> An agent is encouraged to assign monotonically increasing index
> values to new entries, starting with one, after each reboot. It is
> considered unlikely that the lldpRemOrgDefInfoIndex will wrap
> between reboots.

However, it is simpler to implement it this way as we don't need to
record the index inside the `lldpd_custom` structure. Also, the index
will increase even for a different OUI or subtype as we do not want to
sort the custom TLVs.

Fix #330.