ifconfig: fix incorrect wg allowed-ips netmask

ifconfig: fix incorrect wg allowed-ips netmask

Currently when peer information is displayed with `ifconfig wgN peer ..`
or `ifconfig wgN peer-list`, the netmask of the first `allowed-ips` will
be used as the netmask of all CIDR in `allowed-ips`.  For example, if
the list is `192.168.1.0/24, 172.16.0.0/16`, it will display as
`192.168.1.0/24, 172.16.0.0/24`. While this does not affect the actual
functionality, it is very confusing.

Submitted by:   Michael Chiu <nyan -at- myuji.xyz>
Reviewed by:	grehan
Differential Revision: https://reviews.freebsd.org/D28655
Approved by:    re (gjb)

(cherry picked from commit be2003ccfa1b12f372fa1329c38c29fe035f232f)