sdatafile - return the RMSfile name
Sdatafile returns the name of the RMSfile associated with the
form specified by the formname parameter. If there is more than
one form in the formfile whose name is formname, sdatafile uses
the first one found.
Sdatafile returns a NULL pointer if the form name cannot be
Sdatafile is only available in user edit routines.
Chapter 3, "Form Programming Guide"