utf8.h: Add macro TWO_BYTE_UTF8_TO_UNI()

utf8.h: Add macro TWO_BYTE_UTF8_TO_UNI()

The code to do this isn't obvious, as it was wrong in 5 different places
in two different files (forgetting one or both of the required
conversions to UTF (which is a no-op except on EBCDIC machines, or it
would have been detected sooner.)

Some of that code depended on left shifting being truncated in a U8.
This adds UTF_START_MASK so it can work in a larger width variable.