Gdbm_Fetch
#Parent Gdbm_
=>	R0 =	file handle from <Gdbm_Open> or <Gdbm_OpenIn>
	R1 =	pointer to keyword
	R2 =	length of keyword
	R3 =	buffer for data
	R4 =	length of buffer
<=	R0 =	length of data read, or 0 if key not found

Retrieves data filed under a keyword in the database.