- The manpage for umount says that -R can only be used with
a mount point (e.g. /usr/local) and not a device name;
this makes sense because a device might be mounted in multiple
locations, but the mountpoint (and things mounted under it) lives
in the filesystem tree.
- Existing code tried to unmount -R the device, not the mount point,
and so always failed; leaving things mounted that shouldn't.
- because mount() returns an exit code, and 0 is "success",
the if (!code) was backwards: when mounting succeeded, the
TemporaryMount object thought it failed.
- This leads to temp-mounts being left *all over* the place
from os-prober and fstab-handling.
- Most of the time the working dir and stdin are not important,
you just want to run a command in the host, so simplify that
by providing a suitable overload.
- Use that overload from the partition service (for mount and sync).
