[CM] interrupt CLM computation

[Bill Schottstaedt]

> but it won't interrupt a CLM instrument, at least I could find no way to stop pvoc. 

(I clicked send on the previous message just as yours was arriving...)

This looks familiar -- I think sbcl (and maybe others) say you're not
supposed to mess with signals, but my recollection is that sigusr1
or whatever it's called is safe.  I'll have to poke in some musty old
closets...

pvoc.ins does try to trap sigint, but you're right that it is not
seeing it.