add support for table name in SRv6 End.DT* behaviors

add support for table name in SRv6 End.DT* behaviors

it allows to specify also the table name in addition to the table number in
SRv6 End.DT* behaviors.

To add an End.DT6 behavior route specifying the table by name:

    $ ip -6 route add 2001:db8::1 encap seg6local action End.DT6 table main dev eth0

The ip route show to print output this route:

    $ ip -6 route show 2001:db8::1
    2001:db8::1  encap seg6local action End.DT6 table main dev eth0 metric 1024 pref medium

The JSON output:
    $ ip -6 -j -p route show 2001:db8::1
    [ {
            "dst": "2001:db8::1",
            "encap": "seg6local",
            "action": "End.DT6",
            "table": "main",
            "dev": "eth0",
            "metric": 1024,
            "flags": [ ],
            "pref": "medium"
        } ]

Signed-off-by: Paolo Lungaroni <paolo.lungaroni@cnit.it>
Signed-off-by: David Ahern <dsahern@gmail.com>