Create a mechanism for encoding a system errno into the IIC_Exxxxx space.

Errors are communicated between the i2c controller layer and upper layers
(iicbus and slave device drivers) using a set of IIC_Exxxxxx constants which
effectively define a private number space separate from (and having values
that conflict with) the system errno number space. Sometimes it is necessary
to report a plain old system error (especially EINTR) from the controller or
bus layer and have that value make it back across the syscall interface
intact.

I initially considered replicating a few "crucial" errno values with similar
names and new numbers, e.g., IIC_EINTR, IIC_ERESTART, etc. It seemed like
that had the potential to grow over time until many of the errno names were
duplicated into the IIC_Exxxxx space.

So instead, this defines a mechanism to "encode" an errno into the IIC_Exxxx
space by setting the high bit and putting the errno into the lower-order
bits; a new errno2iic() function does this. The existing iic2errno()
recognizes the encoded values and extracts the original errno out of the
encoded value. An interesting wrinkle occurs with the pseudo-error values
such as ERESTART -- they aleady have the high bit set, and turning it off
would be the wrong thing to do. Instead, iic2errno() recognizes that lots of
high bits are on (i.e., it's a negative number near to zero) and just
returns that value as-is.

Thus, existing drivers continue to work without needing any changes, and
there is now a way to return errno values from the lower layers. The first
use of that is in iicbus_poll() which does mtx_sleep() with the PCATCH flag,
and needs to return the errno from that up the call chain.

Differential Revision: https://reviews.freebsd.org/D20975