Improve filesystem mount functionality

This PR improves the `mcv.filesystem` mounting functionality:

1. `mount()` now no-ops if already mounted, unless `remount=True`,
  in which case it will remount.  But it's now side-effect-free by default.

2. You can now call `mount_status()` and/or `mounted(mount_point)` to
  test whether a mount is mounted.  This helps no-op work if things are
  already mounted and ready to go.