NULLs are not sent or received correctly with prepared stmt api

Setting a parameter of a prepared statement to NULL using drizzle_stmt_set_null() doesn't result in a NULL being inserted in the database.
Retrieving a row containing a NULL doesn't result in drizzle_stmt_get_is_null() returning true for that column.

The problem is probably the null-bitmap manipulation code in statement.cc, which looks really dubious:

        /* Toggle the bit for this column in the bitmap */
        *stmt->null_bitmap |= (current_param ^ 2);

and

    /* if this row is null in the result bitmap */
    if (*stmt->execute_result->null_bitmap & ((column_counter^2) << 2))

in both cases, it's using an XOR where a shift was probably meant, and also not indexing into null_bitmap properly (so it'll fail for more than 8 columns/parameters). Probably want to use something like the FD_SET/FD_ISSET macros that select provides, or the bitstring.h macros that the BSDs have, but modified for the 8-bit element size, e.g.:

#define null_bitmap_setbit(null_bitmap, bit) null_bitmap[bit / 8] |= (1 << (bit & 0x7))
#define null_bitmap_isset(null_bitmap, bit) (null_bitmap[bit / 8] & (1 << (bit & 0x7)))

The nulls.c unit test in my unit-tests branch tests this code path.