ifconfig(8): Support to filter output by interface groups
Add options '-g <match_group>' and '-G <no_group>' to select and
unselect interfaces by groups in the output of 'ifconfig -a', just
like the existing '-d' and '-u' options to select only interfaces
that are down or up, respectively. Note that '-g' and '-G' options
can be used at the same time to apply both conditions and their
arguments may contain shell patterns.
Examples:
* To exclude loopback from the list:
ifconfig -a -G lo
* To list interfaces whose group names begin with 't':
ifconfig -a -g 't*'
Obtained from FreeBSD (revision 361790; review D25029)