Limit wireguard peers listing to -A or wg-interface

Limit wireguard peers listing to -A or wg-interface

ifconfig(8) output can get too long when always printing `wgpeers' for all
wg(4) interfaces, so omit it output is requested and/or output is limited
to the interface group "wg" or a specific interface "wgX".

No install media size change as wireguard code is under #ifndef SMALL.

Diff from Mikolaj Kucharski <mikolaj AT kucharski DOT name>
makes Hrvoje Popovski happy
manual bits from jmc
OK sthen