This allows a run-time change of the active vlan, if CONFIG_VLAN is
active at build time.  The pfilter rules are being rewritten every
time the vlan is changed (and we have one VLAN only for the CPU).

   wrc# vlan set 0
   0 ("0") out of range
   Command "vlan": error -22
   wrc# vlan set pippo
   0 ("pippo") out of range
   Command "vlan": error -22

   wrc# vlan
   current vlan: 10 (0xa)

   wrc# vlan set 20
   current vlan: 20 (0x14)
   wrc# ip set 192.168.20.2
   IP-address: 192.168.20.2 (static assignment)

      (and the node now replies to ping on vlan 20)

   wrc# vlan set 10
   current vlan: 10 (0xa)
   wrc# ip set 192.168.10.2
   IP-address: 192.168.10.2 (static assignment)

      (and the node now replies to ping on vlan 10)
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>