namespace: make MountFlags=shared work again

namespace: make MountFlags=shared work again

Since commit 0722b359342d2a9f9e0d453875624387a0ba1be2, 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.