Fix commands during analysis (MultiPV option!)

Fix commands during analysis (MultiPV option!)

UCI2WB could not handle spurious commands while analyzing: these would
start a new search without first terminating the old one. Only the commands
allowed in analysis mode (exit, usermove, and in/exclude) were handled
correctly. Unfortunately XBoard sometimes sends spurious 'accepted' commands
during analysis, in violation of the CECP specs. All such non-compliant
commands are now ignored. And the 'option' command will also abort an
ongoing analysis search, to allow altering the MultiPV setting during
analysis. (Something XBoard also does.)