show_info: Add VPN peer lists for fastd/L2TP

This shows the configured peers for fastd and L2TP.
Note that "configured" does not tell whether they are working.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>