core: delegate mountns implicitly when any of pidns/cgns/netns is in use, clean up private cgroupfs mount (#36892)

Fixes #36952

src/shared/mount-setup.c

@@ -52,7 +52,7 @@ typedef struct MountPoint {
         MountMode mode;
 } MountPoint;
 
-bool cgroupfs_recursiveprot_supported(void) {
+static bool cgroupfs_recursiveprot_supported(void) {
         int r;
 
         /* Added in kernel 5.7 */

src/shared/mount-setup.h

@@ -8,5 +8,3 @@ bool mount_point_ignore(const char *path);
 
 int mount_setup_early(void);
 int mount_setup(bool loaded_policy, bool leave_propagation);
-
-bool cgroupfs_recursiveprot_supported(void);