Initializes an IFFHandle struct for a new session of reading or
writing. The direction of the I/O is determined by the rwMode flags
supplied (IFFF_READ or IFFF_WRITE).
INPUTS
iff
pointer to IFFHandle struct.
ewMode
IFFF_READ or IFFF_WRITE
RESULT
error - 0 if successfull, IFFERR_#? elsewise.
NOTES
This function tells the custom stream handler to initialize
by sending it a IFFCMD_INIT IFFStreamCmd.