postscript: update Runeerror
 [rsc] --rw-rw-r-- M 177229 glenda sys 376 Apr  1 14:04 sys/src/cmd/postscript/common/rune.h
	/n/sourcesdump/2006/0401/plan9/sys/src/cmd/postscript/common/rune.h:14,19 - 
	/n/sourcesdump/2006/0402/plan9/sys/src/cmd/postscript/common/rune.h:14,19
	  	UTFmax		= 3,		/* maximum bytes per rune */
	  	Runesync	= 0x80,		/* cannot represent part of a utf sequence (<) */
	  	Runeself	= 0x80,		/* rune and utf sequences are the same (<) */
	- 	Runeerror	= 0x80,		/* decoding error in utf */
	+ 	Runeerror	= 0xFFFD,		/* decoding error in utf */
	  };
	  #endif