gdb/python: allow for catchpoint type breakpoints in python
This commit adds initial support for catchpoints to the python
breakpoint API.
This commit adds a BP_CATCHPOINT constant which corresponds to
GDB's internal bp_catchpoint. The new constant is documented in the
manual.
The user can't create breakpoints with type BP_CATCHPOINT after this
commit, but breakpoints that already exist, obtained with the
`gdb.breakpoints` function, can now have this type. Additionally,
when a stop event is reported for hitting a catchpoint, GDB will now
report a BreakpointEvent with the attached breakpoint being of type
BP_CATCHPOINT - previously GDB would report a generic StopEvent in
this situation.