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.

is_ptr cannot be used because it follows the chain of types into the 
type which has the context_handle attribute, which is typically "void *" 
and so causes these context handles to be incorrectly detected as 
context handles. Instead, we can use is_aliaschain_ptr to follow the 
chain of aliases without following pointers and the absence of the 
context_handle attribute indicates that it must be present on a type 
after following a pointer.

widl: Use is_string_type for detecting strings in write_typeformatstring_var to make it consistent with write_remoting_arg.

Fix the is_string_type function used for detecting strings by only 
examining aliases instead of both aliases and pointers. This is due to 
the requirement that pointers to strings be handled as pointers and so 
not detected as strings.

shell32: Support getting the shell folder path for users other than the current user and the default user.