Improve the documentation for the os.P_* constants used with the os.spawn*()

Improve the documentation for the os.P_* constants used with the os.spawn*()
functions to include information about how they affect the operation of
those functions when used as the "mode" parameter.
This closes SF bug #468384.

Added warnings to the os.tempnam() and os.tmpnam() functions regarding their
security problem.  These warning mirror the warnings added to the runtime
by Skip Montanaro.