utf8.h: Clean up and use START_MARK definition

utf8.h: Clean up and use START_MARK definition

The previous definition broke good encapsulation rules.  UTF_START_MARK
should return something that fits in a byte; it shouldn't be the caller
that does this.  So the mask is moved into the definition.  This means
it can apply only to the portion that creates something larger than a
byte.  Further, the EBCDIC version can be simplified, since 7 is the
largest possible number of bytes in an EBCDIC UTF8 character.