gdb/python: use return values of add_setshow functions in add_setshow_generic

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.

Change-Id: I04d50736e1001ddb732d81e088468876df9c88ff