Originally, nm-applet loaded the vpn plugins by passing the filename
to g_module_open(). Thereby, g_module_open() allowed for missing file
extension and tries to complete the name with a system-dependent suffix.

When porting to libnm, we kept that behavior but did more elaborate
checks on the file, like checking owner and permissions.

Change to no longer trying to append the system suffix, but require
an exact path. That is no usability problem, because the plugin path
is specified in the .name files, and we just require them now to be the
full path (including the .so extension).
Note also, that this only affects new, libnm-based vpn plugins, thus there
is no change in behavior for legacy libnm-glib based plugins.

Fixes: eed0d0c5