ifconfig: set by default FCC regulatory domain for wireless interfaces.

Change default regulatory domain from DEBUG (no limitations;
exposes all device channels) to FCC; as a result, newly created wireless
interface with default settings will have less chances to violate
country-specific regulations.

This change will not affect drivers with pre-initialized regdomain
structure (currentry ath(4) and mwl(4)); in that case, the default
channel list must correspond to the default regdomain / country setting.

You can switch to another regdomain / country via corresponding
ifconfig(8) options; the driver must implement ic_getradiocaps()
method to restore full channel list.

Full country / regdomain list may be obtained via
'ifconfig <iface> list countries' command.

Example: change country to Germany:
ifconfig wlan0 down # all wlans on the device must be down
ifconfig wlan0 country DE
ifconfig wlan0 up

wpa_supplicant(8), dhclient(8) etc

At the creation time:
ifconfig wlan0 create wlandev wpi0 country DE

To make changes permanent add the following line to the rc.conf(5):
create_args_wlan0="country DE"

Tested with

• Intel 3945BG (wpi(4)).
• WUSB54GC (rum(4)).

Reviewed by: adrian
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D6228