posix: do not define _PTHREAD_CANCEL_POS

_PTHREAD_CANCEL_POS is an implementation detail and should not
be defined in the global scope. Furthermore, _PTHREAD_CANCEL_POS
uses a reserved identifier (underscore followed by capital
letter).

Adjust definitions so that the implementation detail is only
used in the implementation and not in the interface.

Additionally, modify naming so that the non-standard macro does
not use a reserved identifier.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>