gh-119349: Add ctypes.util.dllist -- list loaded shared libraries (GH-122946)

Add function to list the currently loaded libraries to ctypes.util

The dllist() function calls platform-specific APIs in order to
list the runtime libraries loaded by Python and any imported modules.
On unsupported platforms the function may be missing.


Co-authored-by: Eryk Sun <eryksun@gmail.com>
Co-authored-by: Peter Bierma <zintensitydev@gmail.com>