Add ParamValueList::find methods.

Similar to what used to be in the implementation of
ImageSpec::find_attribute, but it seems like it's more appropriate to be
right in the PVL.
Fix image_view::AutoStride -- it was unwisely evaluating to -1
Somebody might actually want that as a stride.
Seems safe to use PTRDIFF_MIN, which is impossible to use as a meaningful
stride.