Scrap readrune(), introducing fgetrune()

Interface as proposed by cls, but internally rewritten after a few
considerations.
The code is much shorter and to the point, aligning itself with other
standard functions. It should also be much faster, which is not bad.

unexpand.c

@@ -73,7 +73,7 @@ unexpand(const char *file, FILE *fp)
 	size_t last = 0, col = 0, i;
 	int bol = 1;
 
-	while (readrune(file, fp, &r)) {
+	while (efgetrune(&r, fp, file)) {
 		switch (r) {
 		case ' ':
 			if (!bol && !aflag)