^Z should be bindable to commands.

[nelhage@mit.edu]

Right now, ^{Z (aka SIGTSTP) is always caught and turned into an error telling you to use ':suspend' to suspend. We should instead catch SIGTSTP and turn it into a fake keystroke like we do for SIGINT, or possibly just disable VSUSP and bind} Z to "error blah-blah-blah", so that users can redefine ^{Z if they choose.}