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.h

@@ -62,6 +62,7 @@ int path_is_fs_type(const char *path, statfs_f_type_t magic_value);
 
 bool is_temporary_fs(const struct statfs *s) _pure_;
 int fd_is_temporary_fs(int fd);
+int fd_is_network_ns(int fd);
 int path_is_temporary_fs(const char *path);
 
 /* Because statfs.t_type can be int on some architectures, we have to cast