ioctls for gnome-vfs

[Alexander Larsson <alexl redhat com>]

Whenever I want to use gnome-vfs for something just a little bit unusual, 
something that maps well to file handling, but then requires some extra 
function to work I'm hindered by the lack of an ioctl() style call in 
gnome-vfs. I really want to get this in gnome 2.2 so that I can do some 
cool stuff I've been planning[1].

So, I wrote a patch that adds gnome_vfs_file_operation that is useful when 
you want to do a backend specific operation. It is modeled on ioctl, but 
slightly different. It uses strings instead of magic numbers, so we can 
easier avoid collisions (backend-specific operations can be prefixed with 
the module name, we can work out the names for any operations that are 
common between several modules). It also passes just one pointer instead 
of using varargs, because otherwise the async framework wouldn't work with 
it.

The patch is appended to this mail. The last testing part may or may not 
be commited, I just used it to test the stuff. If this is ok for commiting 
I will write docs for all the new APIs. This whole thing is 100% backward 
compat, even for modules compiled against the old module ABI 
(GnomeVFSMethod is versioned).

I'd really like to get this in before the API freeze, so that I can start 
implementing some cool features in separate modules and have them work in 
Gnome 2.2.

[1] An example of cool stuff:
I want to write a cdburn: backend that basically is a virtual 
ram-backed filesystem. If you copy files to it we just store the filename, 
and all operations on it are redirected to the real file. When the user 
have built up his CD by dnd:in in nautilus we just traverse the whole 
filesystem and call 
  gnome_vfs_file_control(handle, "cdburn:get_real_filename", &filename)