Type-C Current is a mechanism defined in the USB Type-C spec for
indicating high-current modes at 5 V using nothing other than a voltage
on the CC line.  The FUSB302B maps the voltage to one of the four ranges
we care about, so it's easy to see if 1.5 A or 3 A is available at 5 V
even in systems that don't support PD.

Now, when the PD Buddy Sink is connected to a system without Power
Delivery support, after all attempts at PD communications fail, it falls
back to Type-C Current.  If the Sink is configured for 5 V, it will
monitor the CC line's voltage to see if enough Type-C Current is
available.  If so, the output is turned on.

It's simpler than I realized.  All that's left to do here is the
fallback mechanism for when the Sink is configured for 5 V.  This means
that yet again, a TODO was replaced with another, less critical TODO.

PD uses odd units for voltages and currents.  Power too, but we don't
work with power (yet).  In the shell, we have to convert to and from
these odd units.  We had been doing this in-place every time, which was
a bit crumby.  Now it's done with macros in pd.h, making prettier and
more maintainable code.

There are still a few things that the standard says we Shall do and we
don't, but it Works For Me™.  I haven't implemented anything with
regards to GiveBack support, but that doesn't matter just yet.  Our
handling of VDMs isn't quite right either.  Anyway, it successfully
negotiates with so-called Split PDO power supplies, which is more than I
can say about some commercial products.