[iobuf] Improve robustness of I/O buffer allocation

Guard against various corner cases (such as zero-length buffers, zero
alignments, and integer overflow when rounding up allocation lengths
and alignments) and ensure that the struct io_buffer is correctly
aligned even when the caller requests a non-zero alignment for the I/O
buffer itself.

Add self-tests to verify that the resulting alignments and lengths are
correct for a range of allocations.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/tests/tests.c

@@ -67,3 +67,4 @@ REQUIRE_OBJECT ( profile_test );
 REQUIRE_OBJECT ( setjmp_test );
 REQUIRE_OBJECT ( pccrc_test );
 REQUIRE_OBJECT ( linebuf_test );
+REQUIRE_OBJECT ( iobuf_test );