create_particular long worm tail vs mkclass

create_particular long worm tail vs mkclass

Similar to ^G of 'I' triggering impossible "mkclass found no class 35
monsters", using a leading substring of "long worm tail" (other than
"l" and "long worm") would trigger impossible "mkclass found no class
59 monsters and kill the fuzzer when it escalates impossible to panic.
Tighten up the substring matching.

^G of '~' wasn't affected; it deliberately creates a long worm rather
than the tail of one.  But it was possible to ask for "long worm tail"
as a specific monster type and then override the switch to long worm
when prompted about whether to force the originally specified critter.
I've added a check to prevent that opportunity to override even though
a tail without a head seemed to be harmless.