| isfd | isam file descriptor |
| mode | either ISMANULOCK or ISAUTOLOCK |
ISTRUE, or ISFALSE, in which case the error code is returned by isErrno, or found in isfd->iserrno
this routine can be used to switch the designated file between manual and automatic locking modes.
when switching from automatic to manual, the current record, if locked, will be released.
future plans include the ability to switch to/from exclusive lock as well, but at present an attempt to do so will result in an EBADARG error.