detect and ignore unmounted systemd automount.

[memchr]

When accessing a mount point specified with the x-systemd.autmount option in the fstab file, it is mounted on demand by the systemd.automount feature.

Currently, duf will attempt to access an unmounted systemd automount point, resulting in a noticeable delay, and in the worst case, if the drive is offline, duf will hang indefinitely because the automount timeout is disabled by systemd by default.

systemd[1]: media-backups.automount: Got automount request for /media/backups, triggered by 741148 (duf)

Should it be a reasonable default for duf to ignore all unmounted systemd automounts?