the "unit stranded" detector should try pulsing the "recompute walkability" toggle

[ab9rf]

There have been reports of DF's walkability map getting corrupted, leading to odd pathing fails. This sometimes show up with DFHack's notifier as a unit being stranded for no apparent reason.

Given that we're detecting the symptoms of this bug already, we could mitigate it by automatically strobing the "recompute walkability" global toggle whenever a unit is detected as stranded; this will force DF to recompute the walkability matrix and possibly unstrand the stranded dwarf.

There should be a cooldown on this because recomputing walkability is expensive and doing it frequently will cause significant lag.