namespace-util: introduce helper for combining unshare() + MS_SLAVE remount

We have multiple places we do these two non-trivial operations together,
let's introduce a unified helper for doing both at once.

src/basic/namespace-util.c

@@ -2,6 +2,7 @@
 
 #include <fcntl.h>
 #include <sys/ioctl.h>
+#include <sys/mount.h>
 
 #include "fd-util.h"
 #include "missing_fs.h"
@@ -169,3 +170,16 @@ int fd_is_network_ns(int fd) {
 
         return r == CLONE_NEWNET;
 }
+
+int detach_mount_namespace(void) {
+
+        /* Detaches the mount namespace, disabling propagation from our namespace to the host */
+
+        if (unshare(CLONE_NEWNS) < 0)
+                return -errno;
+
+        if (mount(NULL, "/", NULL, MS_SLAVE | MS_REC, NULL) < 0)
+                return -errno;
+
+        return 0;
+}

src/basic/namespace-util.h

@@ -7,3 +7,5 @@ int namespace_open(pid_t pid, int *pidns_fd, int *mntns_fd, int *netns_fd, int *
 int namespace_enter(int pidns_fd, int mntns_fd, int netns_fd, int userns_fd, int root_fd);
 
 int fd_is_network_ns(int fd);
+
+int detach_mount_namespace(void);