Issue #16804: Fix 'python -S -m site' failure.

Issue #16804: Fix 'python -S -m site' failure.

This commit fixes a bug in the 'site' module that was causing an exception
to incorrectly be thrown when running 'python -S -m site'.  The problem was
that 'USER_BASE' and 'USER_SITE' were being accessed before they were properly
initialized.  The code has been changed to use 'getuserbase' and
'getusersitepackages' instead so that the initialization always happens.