Partially revert 1036cadcec because UCI protocol
in case of multipv explicitly requires:
for the best move/pv add "multipv 1" in the string when you send the pv.
in k-best mode always send all k variants in k strings together.
Thanks to Justin Blanchard for pointing this out.
No functional change.
Signed-off-by: Marco Costalba <mcostalba@gmail.com>