Cleanup AID handling.

Cleanup AID handling.

- Loops over all valid AID should start with AID_MIN and go up to AID_MAX - 1
   e.g. for (i = AID_MIN; i < AID_MAX; i++)
  If for some reason AID_UNSPEC must be handled make that explicit in the
  for loop.

- aid2afi() now returns an error for AID_UNSPEC since there is no valid
  AFI SAFI combo for AID_UNSPEC.

- Add additional checks for AID_MIN where currently only AID_MAX was checked.
  This affects imsg for route refresh and graceful restart.

- Simplify add-path capability handling. Only the negotiated add_path capa
  sets the flag for AID_UNSPEC to help code to quickly check if any add-path
  is active.

OK tb@