Add utfnlen prototype for chan.c.
 [rsc] --rw-rw-r-- M 284191 glenda sys 5770 Sep  7 10:32 sys/src/9/port/lib.h
	/n/sourcesdump/2005/0907/plan9/sys/src/9/port/lib.h:43,48 - 
	/n/sources/plan9/sys/src/9/port/lib.h:43,49
	  extern	int	chartorune(Rune*, char*);
	  extern	char*	utfrune(char*, long);
	  extern	int	utflen(char*);
	+ extern	int	utfnlen(char*, long);
	  extern	int	runelen(long);
	  
	  extern	int	abs(int);