tree-wide: avoid uninitialized warning on _cleanup_ variables

With some versions of the compiler, the _cleanup_ attr makes it think
the variable might be freed/closed when uninitialized, even though it
cannot happen. The added cost is small enough to be worth the benefit,
and optimized builds will help reduce it even further.

src/network/netdev/tuntap.c

@@ -46,7 +46,7 @@ static int netdev_fill_tuntap_message(NetDev *netdev, struct ifreq *ifr) {
 }
 
 static int netdev_tuntap_add(NetDev *netdev, struct ifreq *ifr) {
-        _cleanup_close_ int fd;
+        _cleanup_close_ int fd = -1;
         TunTap *t = NULL;
         const char *user;
         const char *group;