Add suitable knob ifconfig_<interface>_descr for static interface description.

Document availability of interface descriptions within rc.conf(5).

Approved by:	avg (mentor), mav (mentor)
MFC after:	3 days

etc/network.subr

@@ -122,7 +122,7 @@ ifn_vnet0()
 #
 ifconfig_up()
 {
-	local _cfg _ipv6_opts ifconfig_args
+	local _cfg _ifconfig_descr _ipv6_opts ifconfig_args 
 	_cfg=1
 
 	# Make sure lo0 always comes up.
@@ -214,6 +214,11 @@ ifconfig_up()
 	ifalias $1 link alias
 	ifalias $1 ether alias
 
+	_ifconfig_descr=`get_if_var $1 ifconfig_IF_descr`
+	if [ -n "${_ifconfig_descr}" ]; then
+		${IFCONFIG_CMD} $1 description "${_ifconfig_descr}"
+	fi
+
 	if wpaif $1; then
 		/etc/rc.d/wpa_supplicant start $1
 		_cfg=0		# XXX: not sure this should count

share/man/man5/rc.conf.5

@@ -1616,6 +1616,23 @@ is set to 1.
 .Pp
 Default is
 .Dq Li NO .
+.It Va ifconfig_ Ns Ao Ar interface Ac Ns _descr
+.Pq Vt str
+This assigns arbitrary description to an interface.
+The
+.Xr sysctl 8
+variable
+.Va net.ifdescr_maxlen
+limits its length.
+This static setting may be overridden by commands
+started with dynamic interface configuration utilities
+like
+.Xr dhclient 8
+hooks. The description can be seen with
+.Xr ifconfig 8
+command and it may be exported with
+.Xr bsnmpd 1
+daemon using its MIB-2 module.
 .It Va ifconfig_ Ns Ao Ar interface Ac Ns _ipv6
 .Pq Vt str
 IPv6 functionality on an interface should be configured by