Support value-in-range with <number> keyword

Support value-in-range with <number> keyword

This allows syntax checking to pass for fields like vxlan-ttl/vxlan-tos
which can be a number in a range OR a string value representing a special
meaning (0-255 or "auto", for instance).  Without this, you can only pass
a --syntax-check for such fields if your value is one of those literally
specified because, for instance, "64" is not "auto", "0", or "255":

invalid value "64": valid attribute values: ['0', '255']
info: exit status 1

Note that _applying_ such configuration still works, because netlink's
acceptance criteria are independent of ifupdown2's.