Add support for vlans to rc.d startup scripts

[John Baldwin]

This patch takes the work done for VAP devices for wireless and extends it to 
support adding VLAN interfaces to network devices more easily.  A list of vlan 
interfaces is specified via 'vlans_<foo>'.  If the vlan name is just a number, 
then that number is taken to be a tag and a '<foo>.<tag>' vlan interface 
device is created.  Otherwise, a 'create_args_<vlan>' variable must be defined 
that includes the 'vlan <tag>' in order for the vlan to be successfully 
created.  If one had the following lines in rc.conf for example:

vlans_foo="101 bar vlan100"
create_args_bar="vlan 102"
create_args_vlan100="vlan 104"

Then this would result in the following vlan devices being created during 
boot:

foo.101: flags=8842<BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=3<RXCSUM,TXCSUM>
        ether 00:24:81:40:e3:ef
        media: Ethernet autoselect
        vlan: 101 parent interface: foo
bar: flags=8842<BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=3<RXCSUM,TXCSUM>
        ether 00:24:81:40:e3:ef
        media: Ethernet autoselect
        vlan: 102 parent interface: foo
vlan100: flags=8842<BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=3<RXCSUM,TXCSUM>
        ether 00:24:81:40:e3:ef
        media: Ethernet autoselect
        vlan: 104 parent interface: foo

If one uses 'sh /etc/rc.d/netif start foo' then the vlan devices will be 
created, and if one does 'sh /etc/rc.d/netif stop foo' then the vlan devices 
will be destroyed.

One downside is that similar to the wlan interfaces, the ifconfig output for 
these devices is not displayed as part of the normal boot messages.  Doing so 
would require some way for ifn_start and ifn_stop to communicate their list of 
child interfaces along with create/fail status for each back to the main loop 
in /etc/rc.d/netif.

The patch is at http://www.FreeBSD.org/~jhb/patches/vlans_head.patch and is 
included below.

-- 
John Baldwin