libutf

This is a C89 UTF-8 library, with an API compatible with that of Plan 9's libutf, but with a number of improvements:

• Support for runes beyond the Basic Multilingual Plane.
• utflen and utfnlen cannot overflow on 32- or 64-bit machines.
• chartorune treats all invalid codepoints as though Runeerror.
• fullrune, utfecpy, and utfnlen do not overestimate the length of malformed runes.
• An extra function, charntorune(p,s,n), equivalent to fullrune(s,n) ? chartorune(p,s) : 0.
• Runeerror may be set to an alternative replacement value, such as -1, to be used instead of U+FFFD.

Differences to be aware of:

• UTFmax is 6, though runetochar will never write more than 4 bytes. Plan 9's UTFmax is 3.
• chartorune may consume multiple bytes for each illegal rune. Plan 9 always consumes 1.
• runelen and runetochar return 0 if the rune is too large to print. Plan 9 erroneously returns UTFmax.