Clarify zone names

It transforms zones files like this:
#ZONE                          TYPE       OPTIONS
$FW                            firewall
$ZVMBR0                        ipv4
$ZVMBR0EXT:$ZVMBR0             bport
$ZVMBR0VM100:$ZVMBR0          bport
$ZVMBR0VM101:$ZVMBR0          bport

into this:
#ZONE                          TYPE       OPTIONS
$FW                            firewall
$VMBR0                         ipv4
$VMBR0_EXT:$VMBR0              bport
$VMBR0_VM100:$VMBR0            bport
$VMBR0_VM101:$VMBR0            bport

Signed-off-by: Michel Loiseleur <michel@loiseleur.com>