Closes https://github.com/cython/cython/issues/3590

Test modf, since it's in C89 and has a non-trivial signature.

cimport some C99 float and long double functions, and test legacy kwargs
for double functions.

Adds many long double variants (e.g. expl) that were previously omitted.
No functions have been removed.

I've omitted the identifier for trivial parameters in new functions.
However, I've retained them for existing functions for compatibility
with legacy kwargs.

Classes of functions/macros that are still missing:
 - error-handling ones, like math_errhandling and MATH_ERRNO
 - Bessel functions like j0, y0

Fixes cython/cython#3570

Sorting alphabetically by function name makes it easier to audit
changes.

Also changes "type *" -> "type*"

Closes #3579.

Restore the previous PyThreadState_Get() calls in the refnanny, but with a comment explaining the purpose of the "useless" calls.

Turn all legacy struct fields in the NumPy ndarray type into properties and document them (copied from NumPy docs).
Adapt the 'nogil' declarations according to the ones in NumPy and add some missing functions.
See https://github.com/numpy/numpy/pull/16170

* Support for versioned pxd files like "lib.cython-30.pxd" for a Cython 3.0+ version.
See https://github.com/cython/cython/issues/3573

* Fix test in Py2 by avoiding namespace packages in favour of a normal package.

Remove "PYTHONHOME" setting from embed-test since it seems to break the I/O encoding setup in (at least) Py3.8+ venvs.

See https://github.com/numpy/numpy/pull/16170

Add more 'nogil' declarations to NumPy functions that do not require the GIL (according to the declarations shipped by NumPy itself).
See https://github.com/numpy/numpy/pull/16170

Remove the outdated getbuffer/releasebuffer implementations for the NumPy 'ndarray' since there are probably no NumPy installations out there anymore that do not support the buffer protocol themselves and are still worth supporting.

See https://github.com/numpy/numpy/pull/12284#discussion_r420378155
See GH-3573

Reverse the tuple item order in the utility code caching key to move the most selective item first (just in case if we ever get dict collisions).

See https://github.com/cython/cython/issues/3573

Tighten "numpy_attributes" test to assert that it's actually using a property call and not an attribute access.
Also add a test for the newly added "ndarray.data" property.

Mark the embedded C main() function in the embedding test as "extern C" in C++ to see if that fixes the C++ builds in MSVC.

Fix a C compiler warning about unused cleanup code in buffer using functions that always raise (found by clang in the bufaccess.pyx test).