gh-111089: Add PyUnicode_AsUTF8() to the limited C API (#111121)

Add PyUnicode_AsUTF8() function to the limited C API.

multiprocessing posixshmem now uses PyUnicode_AsUTF8() instead of
PyUnicode_AsUTF8AndSize(): the extension is built with the limited C
API. The function now raises an exception if the filename contains an
embedded null character instead of truncating silently the filename.