csharp: support command line arguments for scripts

csharp: support command line arguments for scripts

This adds a command line argument preprocessor that splits command line
arguments into two groups, delimited by the "standard" `--` "stop
processing" argument, or by the `-s` argument which can be used with
Unix shebang syntax (where the path of the file being executed will
be appended to the command line specified on the shebang line).

- anything before `--` or `-s` will be passed to mcs/csharp as usual
- anything after `--` or `-s` will _not_ be processed by mcs/csharp,
  and will be made available in an `Args` builtin, like `csi`

This _finally_ (after almost 10 years) allows `csharp` to be used for
proper script authoring!

Document the new arguments in the man page. Also remove references to
'gsharp' from the man page.