Introduce ip vrf command

Introduce ip vrf command

'ip vrf' follows the user semnatics established by 'ip netns'.

The 'ip vrf' subcommand supports 3 usages:

1. Run a command against a given vrf:
       ip vrf exec NAME CMD

   Uses the recently committed cgroup/sock BPF option. vrf directory
   is added to cgroup2 mount. Individual vrfs are created under it. BPF
   filter attached to vrf/NAME cgroup2 to set sk_bound_dev_if to the VRF
   device index. From there the current process (ip's pid) is addded to
   the cgroups.proc file and the given command is exected. In doing so
   all AF_INET/AF_INET6 (ipv4/ipv6) sockets are automatically bound to
   the VRF domain.

   The association is inherited parent to child allowing the command to
   be a shell from which other commands are run relative to the VRF.

2. Show the VRF a process is bound to:
       ip vrf id
   This command essentially looks at /proc/pid/cgroup for a "::/vrf/"
   entry with the VRF name following.

3. Show process ids bound to a VRF
       ip vrf pids NAME
   This command dumps the file MNT/vrf/NAME/cgroup.procs since that file
   shows the process ids in the particular vrf cgroup.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>