form_field_userptr - associate application data with a form field
SYNOPSIS
#include <form.h>
int set_field_userptr(FIELD *field, void*userptr);
void *field_userptr(const FIELD *field);
DESCRIPTION
Every form field has a field that can be used to hold application-specific data
(that is, the form-driver code leaves it alone). These functions get and set
that field.
RETURN VALUE
The function field_userptr returns a pointer (which may be NULL).
It does not set errno.
The function set_field_userptr returns E_OK (success).
