python: Use CLOEXEC on script

python: Use CLOEXEC on script

I am unable to quickly determine whether the mere act of loading a
python file can cause enough code to execute as to trigger the Python
interpreter into attempting a fork.  So for safety's sake, let's set
CLOEXEC on the file before handing it to python rather than risking an
accidental fd leak.  Since Haiku lacks fopen("re"), and I already
decided that set_cloexec(fileno(fp)) is undesirable, this has to use
the atomic open/fdopen sequence, even though it does not actually need
atomicity.

Signed-off-by: Eric Blake <eblake@redhat.com>