ENH: intern some commonly used strings in umath module

Allows using the PyObject dictionary functions instead of the costly
C-string variants.

Make use of this in some of the ufunc subtype wrapper functions.

Improvement:

    a, b = np.arange(200.), np.arange(200.)
    %timeit np.add(a, b, out=a)
    1000000 loops, best of 3: 768 ns per loop

vs before:

    %timeit np.add(a, b, out=a)
    1000000 loops, best of 3: 1.03 µs per loop
ENH: skip redundant checking of subok keyword in _find_array_prepare

all currently existing callers only call this function if subok has
already been checked to be true, but keep the code to make clear it has
to be checked.