Add some new subclasses of breakpoint

Add some new subclasses of breakpoint

This adds a few new subclasses of breakpoint.  The inheritance
hierarchy is chosen to reflect what's already present in
initialize_breakpoint_ops -- it mirrors the way that the _ops
structures are filled in.

This patch also changes new_breakpoint_from_type to create the correct
sublcass based on bptype.  This is important due to the somewhat
inverted way in which create_breakpoint works; and in particular later
patches will change some of these entries.