Noncharacters 0xFFFE and 0xFFFF don't render a UTF-8 string invalid

Noncharacters 0xFFFE and 0xFFFF don't render a UTF-8 string invalid
so stop rejecting them in our citrus UTF-8 parser.

This is a common misinterpretation of the Unicode standard which resulted
in a corrigendum last year: http://www.unicode.org/versions/corrigendum9.html

Pointed out by jilles@freebsd (via pfg@freebsd), thanks!