[DotGNU]Attributes in cscc

[Rhys Weatherley]

After much procrastination on my part, attributes are now mostly
working in the C# compiler.  DllImport, MethodImpl, etc are
working fine now.  The major step remaining is named arguments
to attributes. e.g.

    [AttributeUsage(AttributeTargets.All, AllowMultiple=false, Inherited=true)]

("AllowMultiple" and "Inherited" are named arguments).  Ordinary
positional arguments are working.  I hope to have named arguments
working in the next few days.

A metadata-only compile on pnetlib now inserts the correct
"internalcall" designations in the right places, which should
make Gopal a very happy camper.

Cheers,

Rhys.