Use O_PATH when opening parent directories

[smcv]

• Use O_PATH when opening parent directories

  Otherwise we need read permission on every path component from the root
  to the mount point. This isn't available if we are mounting onto the
  $TMPDIR created by pam_tmpdir or pam_mktemp, which create a
  root-owned directory with a+x but not a+r permission, and then a
  per-uid directory inside that.

  Resolves: 0.12.0 regression?: Can't bind-mount a directory whose parent is unreadable #806
  Resolves: Regression for users of pam_tmpdir or pam_mktemp ValveSoftware/steam-runtime#855

• Also add O_CLOEXEC while I'm changing these lines already

  In this case it has no functional effect, because we close these fds
  with cleanup_fd before exec'ing anything (either within this function
  or in its only caller), but it's good to get into a habit of making
  all fds be close-on-exec except for the few that we specifically want
  to allow child processes to inherit.

cc @ao2

[ao2]

The changes look good to me, I cannot approve formally because I am not a reviewer of this PR.

[smcv]

I'll merge this based on my colleague @ao2's review if there's no response from other maintainers for a while.

(I suspect that in practice this mainly affects the Steam Runtime, in which we use a local build of bubblewrap and have cherry-picked these changes.)