When a VPN plugin logs to syslog(), it should not use the syslog
levels that were passed in by NetworkManager directly. Instead,
it must map LOG_NOTICE to LOG_INFO and LOG_INFO to LOG_DEBUG.
Add a utility function does gets that right.
Soon we will add proxy support where VPN plugins set a property
NM_VPN_PLUGIN_CONFIG_PROXY_PAC.
All a VPN plugin needs to make use of this new setting is the
NM_VPN_PLUGIN_CONFIG_PROXY_PAC define.
We don't want that older plugins (still compatible with libnm 1.2 API)
require a new API only for this define. Define it instead in
"shared/nm-utils/nm-vpn-plugin-macros.h" as fallback.
https://mail.gnome.org/archives/networkmanager-list/2016-June/msg00154.html