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/missing.h

@@ -1271,4 +1271,12 @@ struct fib_rule_uid_range {
 #  define EXT4_IOC_RESIZE_FS              _IOW('f', 16, __u64)
 #endif
 
+#ifndef NSFS_MAGIC
+#define NSFS_MAGIC 0x6e736673
+#endif
+
+#ifndef NS_GET_NSTYPE
+#define NS_GET_NSTYPE _IO(0xb7, 0x3)
+#endif
+
 #include "missing_syscall.h"