The check_return argument and ufunc object field was never used; ditto
for the "new" inner loop selector (which was never implemented), along
with associated typedefs. Since I was looking at this code anyway trying
to figure out which parts were actually in use, I figured I'd clear up
some of the brush to make it easier next time...

Examples

:cdata: -> :c:data:
.. cfunction:: -> .. c:function::

Documented the the new behavior in c-api.generalized-ufuncs.rst.

Added PyUFunc_FromFuncAndDataAndSignature to c-api.ufunc.rst.

Clearing is 50-100 times more expensive than checking on x86, so check if there
is anything needs to be cleared first. This speeds up scalar operations
by 10%-20%.
Based on Arink Verma code in #3739.

Implement the functions as new C-API functions npy_get_floatstatus and
npy_clear_floatstatus in npy_math.