```lib/math/rational.c: fix possible incorrect result from rational fractions helper

In some cases the previous algorithm would not return the closest
approximation.  This would happen when a semi-convergent was the closest,
as the previous algorithm would only consider convergents.

As an example, consider an initial value of 5/4, and trying to find the
closest approximation with a maximum of 4 for numerator and denominator.
The previous algorithm would return 1/1 as the closest approximation,
while this version will return the correct answer of 4/3.

To do this, the main loop performs effectively the same operations as it
did before.  It must now keep track of the last three approximations,
n2/d2 ..  n0/d0, while before it only needed the last two.

semi-convergent term, t, which is a single expression with two divisions:

min((max_numerator - n0) / n1, (max_denominator - d0) / d1)

This will be used if it is better than previous convergent.  The test for
this is generally a simple comparison, 2*t > a.  But in an edge case,
where the convergent's final term is even and the best allowable
semi-convergent has a final term of exactly half the convergent's final
term, the more complex comparison (d0*dp > d1*d) is used.

I also wrote some comments explaining the code.  While one still needs to
look up the math elsewhere, they should help a lot to follow how the code
relates to that math.

This routine is used in two places in the video4linux code, but in
those cases it is only used to reduce a fraction to lowest terms, which
the existing code will do correctly.  This could be done more
efficiently with a different library routine but it would still be the
Euclidean alogrithm at its heart.  So no change.

The remain users are places where a fractional PLL divider is
programmed.  What would happen is something asked for a clock of X MHz
but instead gets Y MHz, where Y is close to X but not exactly due to
the hardware limitations.  After this change they might, in some cases,
get Y' MHz, where Y' is a little closer to X then Y was.

Users like this are: Three UARTs, in 8250_mid, 8250_lpss, and imx.  One
GPU in vp4_hdmi.  And three clock drivers, clk-cdce706, clk-si5351, and
clk-fractional-divider.  The last is a generic clock driver and so
would have more users referenced via device tree entries.

I think there's a bug in that one, it's limiting an N bit field that is
offset-by-1 to the range 0 ..  (1<<N)-2, when it should be (1<<N)-1 as
the upper limit.

I have an IMX system, one of the UARTs using this, so I can provide a
real example.  If I request a custom baud rate of 1499978, the driver
will program the PLL to produce a baud rate of 1500000.  After this
change, the fractional divider in the UART is programmed to a ratio of
65535/65536, which produces a baud rate of 1499977.0625.  Closer to the
requested value.