Change __Pyx_zeros and __Pyx_minusones scope to static.

Compiling two Cython modules (which use the Cython buffer interface)
into a static Python binary results in a link error since the symbols
`__Pyx_zeros` and `__Pyx_minusones` are exported multiple times.  Changing
their scope to the current translation unit fixes the issue.

Specifying a custom main function with --embed= causes a TypeError

Fix variable name in doc.

Fix a mismatched variable name in the "Sharing Extention Types"
documentation.

Atomic acquisition counting, slice assignment (+scalars), overlapping memory, broadcasting, more memoryview object methods, optimized with gil block