In gimp_plug_in_open(), use gimp_spawn_set_cloexec() to prevent the
parent's end of the read/write pipes from being inherited by the
spawned plug-in, instead of passing the corresponding file
descriptors to the plug-in as command-line arguments, and having
gimp_main() close them.

Adding new command-line arguments to plug-ins is problematic, since
their ability to handle them depends on their protocol version,
which is only communicated after the plug-in is spawned.

Regardless, this is much simpler.