Make i2c -s (device scan) work on hardware that supports only full xfers.

The existing scan code is based on sending an i2c START condition and if
there is no error it assumes there is a device at that i2c address. Some
i2c controllers don't support sending individual start/stop signals on the
bus, they can only perform complete data transfers with start/stop handled
in the silicon.

This adds a fallback mechanism that attempts to read a single byte from each
i2c address. It's less reliable than looking for an an ACK repsonse to a
start, because some devices will NAK an attempt to read that isn't preceeded
by a write of a register address. Writing to devices to probe them is too
dangerous to even consider. The user is told that a less-reliable scan is
being done, so even if the read-scan comes up empty too, it's still a vast
improvement over the old situation where it would just claim there were no
devices on the bus even though the devices were there and working fine.

If the i2c controller responds with a proper ENODEV (device doesn't support
operation) or an almost-proper EOPNOTSUPP, the START/STOP scan is switched
to a read-scan right away. Most controllers respond with ENXIO or EIO if
they don't support START/STOP, so no quick-out is available. For those,
if a scan of all 127 addresses and come up empty, the scan is re-done using
the read method.

Reported by: Maxim Filimonov <che@bein.link>