Introduce ifconfig -a -g groupname

[Eugene Grosbein]

Hi!

Currently "ifconfig -a" command that shows status of network interfaces
may be combined with flags -d or -u to limit the list to interfaces that are down or up.

The change https://reviews.freebsd.org/D25029 allows it to filter the list
by name of interface group with additional flag -g groupname, or -g ^groupname to negate condition
(this is different from "ifconfig -g groupname" that shows interface names only
and that behaviour is not affected with the change).

I chose caret symbol (^) was choosen to ease both scripted and interactive usage
so it does not require extra quotation/escaping, but was told
that caret would require escaping in the zsh.

So I ask for suggestions which symbol to choose instead of caret.
Benedict Reuschling suggested @ and I'm fine with it
if we don't care about Perl code that would require escaping it when running shell code.

For thouse who interested, these are supposed usage 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