Resume after power fail when print is from external SD card (standalone mode)

[dc42]

If power fails when printing in standalone mode from an external SD card (e.g. on PanelDue) then resuming the print fails unless the SD card is mounted prior to running M916, or M21 P1 is added to resurrect-prologue.g. See https://forum.duet3d.com/topic/33515/resume-apf-not-working.

We should either mount the external SD card automatically if the resumed file is not on card 0, or (preferable) auto-mount a card any time a file request is made on it.