Add checks when reading VRPs with a maxLength via RTR

Add checks when reading VRPs with a maxLength via RTR

Ensure the maxLength is greater than or equal to the length of
the accompanying prefix, and less than or equal to the length
(in bits) of an IP address in the address family (32 for IPv4
and 128 for IPv6). The same checks are applied when reading VRPs
from configuration on disk.

OK claudio@