altos: Make stdio 8-bit clean by making pollchar return int

We were stealing one value (0xff) in the return value from pollchar to
indicate 'not ready yet'. Instead of doing that, use the integer value
-1 and have pollchar return an int instead of a char. That
necessitated cleaning a few other bits to make sure that 0xff wouldn't
get promoted to -1 on accident.

Signed-off-by: Keith Packard <keithp@keithp.com>