In gimp_plug_in_open(), use gimp_spawn_async(), added in the
previous commit, instead of g_spawn_async().  See the previous
commit for the rationale.

Since gimp_spawn_async() doesn't provide a mechanism to perform any
cleanup in the child before exec()ing, move the closing of the
parent's end of the read/write pipes from the app to the plug-in's
gimp_main(), passing the relevant file descriptors to the plug-in
through argv.