gdb/guile: allow for catchpoint type breakpoints in guile
This commit adds initial support for catchpoints to the guile
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 (breakpoints) function, can now have this type.