| isfd | isam file descriptor |
| kdsc | pointer to dictinfo or key description structure |
| idx | zero for dictinfo, otherwise index number |
ISTRUE, or ISFALSE, in which case the error code is returned by isErrno, or found in isfd->iserrno
this call serves two somewhat different purposes.
if idx is zero, then it is actually expecting a pointer to a dictinfo struct, which will then be loaded with information about the current file. this is a C-ISAM inherited oddness, please see isIsamInfo for a somewhat tidier method.
if idx is non zero then isIndexInfo will return the specified key description, where a value of 1 represents the primary, followed by each successive index in the file.