nspawn: introduce an option for specifying network namespace path

Add a new option `--network-namespace-path` to systemd-nspawn to allow
users to specify an arbitrary network namespace, e.g. `/run/netns/foo`.
Then systemd-nspawn will open the netns file, pass the fd to
outer_child, and enter the namespace represented by the fd before
running inner_child.

```
$ sudo ip netns add foo
$ mount | grep /run/netns/foo
nsfs on /run/netns/foo type nsfs (rw)
...
$ sudo systemd-nspawn -D /srv/fc27 --network-namespace-path=/run/netns/foo \
  /bin/readlink -f /proc/self/ns/net
/proc/1/ns/net:[4026532009]
```

Note that the option `--network-namespace-path=` cannot be used together
with other network-related options such as `--private-network` so that
the options do not conflict with each other.

Fixes https://github.com/systemd/systemd/issues/7361

src/basic/stat-util.c

@@ -226,6 +226,18 @@ int fd_is_temporary_fs(int fd) {
         return is_temporary_fs(&s);
 }
 
+int fd_is_network_ns(int fd) {
+        int r;
+
+        r = fd_is_fs_type(fd, NSFS_MAGIC);
+        if (r <= 0)
+                return r;
+        r = ioctl(fd, NS_GET_NSTYPE);
+        if (r < 0)
+                return -errno;
+        return r == CLONE_NEWNET;
+}
+
 int path_is_temporary_fs(const char *path) {
         _cleanup_close_ int fd = -1;