namespace: make MountFlags=shared work again

Since commit 0722b35934, the root mountpoint is
unconditionnally turned to slave which breaks units that are using explicitly
MountFlags=shared (and no other options that would implicitly require a slave
root mountpoint).

Here is a test case:

  $ systemctl cat test-shared-mount-flag.service
  # /etc/systemd/system/test-shared-mount-flag.service
  [Service]
  Type=simple
  ExecStartPre=/usr/bin/mkdir -p /mnt/tmp
  ExecStart=/bin/sh -c "/usr/bin/mount -t tmpfs -o size=10M none /mnt/tmp && sleep infinity"
  ExecStop=-/bin/sh -c "/usr/bin/umount /mnt/tmp"
  MountFlags=shared

  $ systemctl start test-shared-mount-flag.service
  $ findmnt /mnt/tmp
  $

Mount on /mnt/tmp is not visible from the host although MountFlags=shared was
used.

This patch fixes that and turns the root mountpoint to slave when it's really
required.

src/core/execute.c

@@ -1839,7 +1839,7 @@ static bool exec_needs_mount_namespace(
         if (context->n_temporary_filesystems > 0)
                 return true;
 
-        if (context->mount_flags != 0)
+        if (!IN_SET(context->mount_flags, 0, MS_SHARED))
                 return true;
 
         if (context->private_tmp && runtime && (runtime->tmp_dir || runtime->var_tmp_dir))
@@ -2435,6 +2435,9 @@ static int apply_mount_namespace(
         else
                 ns_info = (NamespaceInfo) {};
 
+        if (context->mount_flags == MS_SHARED)
+                log_unit_debug(u, "shared mount propagation hidden by other fs namespacing unit settings: ignoring");
+
         r = setup_namespace(root_dir, root_image,
                             &ns_info, context->read_write_paths,
                             needs_sandboxing ? context->read_only_paths : NULL,