ifconfig(8): make it possible to filter output by interface group.

Now options -g/-G allow to select/unselect interfaces by groups
in the "ifconfig -a" output just like already existing -d/-u.

Examples:

to exclude loopback from the list: ifconfig -a -G lo
to show vlan interfaces only: ifconfig -a -g vlan
to show tap interfaces that are up: ifconfig -aug tap

Arguments to -g/-G may be shell patterns and both may be specified.
Later options -g/-G override previous ones.

MFC after: 2 weeks
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D25029