Fix #97: Use UD1 instruction instead of INT3
For lisp code use the UD1 instruction instead of INT3. For breakpoints we continue to use the INT3 instruction but don't need to insert the breakpoint trap code since int3 cannot come from lisp code anymore.
This also makes working with gdb a bit easier, as long as you don't enable breakpoints in lisp while debugging with gdb.
Main UD2 changes from Carl with other updates from Raymond for using UD1 instead of UD2 and making breakpoints work with UD1.