=> 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.