widl: Make sure that string types are always written out by write_string_tfs and not write_simple_pointer.

This is because write_string_tfs has support for the full range of 
string types, write_simple_pointer doesn't have access to the var 
attributes which are needed to properly detect strings, and 
write_string_tfs sets the typestring offset to after the pointer is 
written and write_remoting_arg depends on this.