sd-device/networkd: unify code to get a socket for issuing netdev ioctls on

As suggested here:

https://github.com/systemd/systemd/pull/4296#issuecomment-251911349

Let's try AF_INET first as socket, but let's fall back to AF_NETLINK, so that
we can use a protocol-independent socket here if possible. This has the benefit
that our code will still work even if AF_INET/AF_INET6 is made unavailable (for
exmple via seccomp), at least on current kernels.

src/basic/socket-util.c

@@ -1060,3 +1060,20 @@ struct cmsghdr* cmsg_find(struct msghdr *mh, int level, int type, socklen_t leng
 
         return NULL;
 }
+
+int socket_ioctl_fd(void) {
+        int fd;
+
+        /* Create a socket to invoke the various network interface ioctl()s on. Traditionally only AF_INET was good for
+         * that. Since kernel 4.6 AF_NETLINK works for this too. We first try to use AF_INET hence, but if that's not
+         * available (for example, because it is made unavailable via SECCOMP or such), we'll fall back to the more
+         * generic AF_NETLINK. */
+
+        fd = socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC, 0);
+        if (fd < 0)
+                fd = socket(AF_NETLINK, SOCK_RAW|SOCK_CLOEXEC, NETLINK_GENERIC);
+        if (fd < 0)
+                return -errno;
+
+        return fd;
+}