iic: return errnos to userspace instead of IIC_ error numbers

[val_packett.cool]

Summary

Before:

% i2c -f /dev/iic1 -a 0x20 -d r -c 513
i2c: ioctl(I2CRDWR) failed: Exec format error

After:

% i2c -f /dev/iic1 -a 0x20 -d r -c 513
i2c: ioctl(I2CRDWR) failed: Operation not supported

---

(the example above is IIC_ENOTSUPP due to an overly long read)

So yeah, turns out this has been a thing for years :/

I guess we could alternatively keep the current way, document it in the manpage and add an iic_strerror to i2c(8) (which would also require moving the IIC_E* definitions to a header shared with userspace) but since nobody noticed that the errors are weird, it should be fine to change.

[avg]

I am a bit torn on this.
On the one hand, I like the simplicity that comes from returning errno codes to userland.
On the other hand, recently I had to write a program that had to use "low level" ioctl-s like I2CSTART and I needed to be able to distinguish IIC_ENOACK from other bus conditions.
In any case, the current state of matters where a return value can be either an errno code or an IIC code is clearly a mess.

[val_packett.cool]

Yeah, we should probably make the iic-errnos public instead. In freebsd-embedded-hal the API I'm implementing also prefers more specific errors.