gh-78502: Add a trackfd parameter to mmap.mmap() (GH-25425)

If *trackfd* is False, the file descriptor specified by *fileno*
will not be duplicated.

Co-authored-by: Erlend E. Aasland <erlend@python.org>
Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>