gsubprocesslauncher: Allow to close FDs

By default, when using g_subprocess_launcher_take_fd() to pass an FD to a child, the GSubprocessLauncher object also takes ownership of the FD in the parent, and closes it during finalize(). This is a reasonable assumption in the majority of the cases, but sometimes it isn't a good idea.

An example is when creating a GSubprocessLauncher in JavaScript: here, the destruction process is managed by the Garbage Collector, which means that those sockets will remain opened for some time after all the references to the object has been droped. This means that it could be not possible to detect when the child has closed that same FD, because in order to make that work, both FDs instances (the one in the parent and the one in the children) must be closed. This can be a problem in, as an example, a process that launches a child that communicates with Wayland using an specific socket (like when using the new API MetaWaylandClient).

Of course, it isn't a valid solution to manually call close() in the parent process just after the call to spawn(), because the FD number could be reused in the time between it is manually closed, and when the object is destroyed and closes again that FD. If that happens, it will close an incorrect FD.

One solution could be to call run_dispose() from Javascript on the GSubprocessLauncher object, to force freeing the resources. Unfortunately, the current code frees them in the finalize() method, not in dispose() (this is fixed in !1670 (merged) ) but it isn't a very elegant solution.

This proposal adds a new method, g_subprocess_launcher_close(), that allows to close all the FDs passed to the child process. To avoid problems, after closing an FD with this method, no more spawns are allowed.