Process#exec: set close-on-exec to false for fd redirection

[XrXr]

This is specifically for redirecting fd => fd.

Process#spawn behaves similarly, but that is more involved
to implement. MRI does it by setting close-on-exec after fork
and before exec while the current spawn implementation uses
posix_spawnp, which spawns in one go and offers less flexibility.

Shopify#1

[eregon]

Process#spawn behaves similarly, but that is more involved
to implement. MRI does it by setting close-on-exec after fork
and before exec while the current spawn implementation uses
posix_spawnp, which spawns in one go and offers less flexibility.

We could do something like that too, by always going through lib/truffle/spawn-helper, but I'm not sure what would be the downsides of that (potentially slower and some potential races until the helper executed the various close/fcntl.

We could also technically use fork+exec in native code in libtruffleposix.so, but that would be a large change.