shared/compat: fix memory handling of nm_setting_vpn_get_*_keys()

The previous fix was bad because the keys do not come from NMSettingVpn's hash
table but are copies that are freed by nm_setting_vpn_foreach_* before
it returns.

[thaller@redhat.com: import shared code from NetworkManager, merging
three patches together.]

Fixes: e93ca7fc
Fixes: 0c90e08f

https://mail.gnome.org/archives/networkmanager-list/2017-December/msg00069.html
https://mail.gnome.org/archives/networkmanager-list/2017-December/msg00070.html