[3.13] gh-143304: Fix ctypes.CDLL to honor handle parameter on POSIX systems (GH-143318) (GH-145173)

The handle parameter was being ignored in the POSIX implementation
of CDLL._load_library(), causing it to always call _dlopen() even
when a valid handle was provided. This was a regression introduced
in recent refactoring.
(cherry picked from commit 27ded243485670fa836c9bb421e37a6ef16eca8e)

Co-authored-by: Arjit Singh Grover <143692910+Koolvansh07@users.noreply.github.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>