PR5197: arg: Allow flags such as --flag=arg as well as --flag arg.

Allow flags to be followed directly by their argument, separated by an '=' sign.  This is consistent with what GNU getopt_long and many other command line parsing libraries allow.

Fix for the following issue:
http://caml.inria.fr/mantis/view.php?id=5197

Bug here:
```
./a.out -foo=blah y z
```
The `y` is never passed to `anon_fun`.

You need to move the `incr current` instruction into the `None` branch of `get_arg`.
It would also be nice to add a few tests for this feature (and, if you have the time, for the `Arg` module in general).