Convert humansize() to accept a size_t instead of a double

General convention is to use size_t to store sizes of all kinds.
Internally, the function uses double anyway, but at least this
doesn't clobber up the API any more and there's a chance in the
future to make this function a bit cleaner and not use this dirty
static buffer hack any more.

ls.c

@@ -162,7 +162,7 @@ output(const struct entry *ent)
 	printf("%s %4ld %-8.8s %-8.8s ", mode, (long)ent->nlink, pwname, grname);
 
 	if (hflag)
-		printf("%10s ", humansize((unsigned long)ent->size));
+		printf("%10s ", humansize(ent->size));
 	else
 		printf("%10lu ", (unsigned long)ent->size);
 	printf("%s %s%s", buf, ent->name, indicator(ent->mode));