ensure we have a ponder move in forced move cases
depth >= 2 simply ensures that (in most cases) we have a non null pv[1].
of course, if the search is aborted (time, depth, nodes etc.) by an AbortSearch exception, before depth 2 is finished, it takes precedence.
namespace search
which obviously conflicts with the function search(), so rename it to pvs(). after all it's just a PVS algorithm (with a few refinments or rather hacks).