gdb/python: use return values of add_setshow functions in add_setshow_generic
In add_setshow_generic, we create set/show commands using add_setshow_*
functions, then look up the commands by name to set the context pointer.
It would be simpler and more efficient to use the return values of the
add_setshow_* functions, do that.
gdb/ChangeLog:
* python/py-param.c (add_setshow_generic): Use return values of
add_setshow functions.