GNetworkMonitorNetlink: make the netlink socket cloexec

Use the same code GSocket does, to try SOCK_CLOEXEC first, and then
fall back to FD_CLOEXEC if it fails. (And fix that code to not call
fcntl if SOCK_CLOEXEC worked.)

https://bugzilla.gnome.org/show_bug.cgi?id=692332