Motor: allow position restore when cannot redefine hardware position

EPICS motor boot time restore seems a zero in DVAL and then resets motor position (DVAL) to its autosaved value. In cases when DVAL cannot be updated in hardware (but goes to zero), can we do an alternative restore where we effectively restore using variabel offsets i.e.

new_OFF = DIR * old_DVAL + old_OFF

this uses off to compessate for the change in DVAL rather than changing DVAL / writing to hardware. It is like updating VAL when variable offset mode is enabled in motor record

there may be subtleties in this. DVAL is restored at autosave psss 0 so available in the driver but not set again. Other fields may be set in pass0 and pass1, so e.g. if the position restore updated OFF it might get overwritten by the previous autosaved value when it applies pass1. Needs some investigation.