Gdbm_NextKey
#Parent Gdbm_
=>	R0 =	file handle from <Gdbm_Open>or <Gdbm_OpenIn>
	R1 =	pointer to last keyword
	R2 =	length of keyword
	R3 =	pointer to buffer
	R4 =	length of buffer
<=	R0 =	length of keyword, or -1 if no keywords exist

Returns the next keyword from the database.

NOTE: Keywords are ordered in a hash table order. This is not necessarily alphabetic, and should not be relied on if <Gdbm_Delete> is called in the midst of a NextKey loop.
