Move the code for determining monster class from user's input string
out of do_class_genocide() and into new routine name_to_monclass(). I'm
planning to use it when name_to_mon fails to match anything for controlled
polymorph (not ready for prime time yet).
Also, avoid getting stuck in a loop if hangup occurs while prompting
player for class of monster to genocide. ESC, whether deliberate or fake
input after hangup, will now be the same as specifying "none", throwing
away the genocide opportunity.
