Add weprintf() and replace fprintf(stderr, ...) calls

There is still some programs left to be updated for this.

Many of these programs would stop on the first file that they
could not open.

head.c

@@ -1,5 +1,4 @@
 /* See LICENSE file for copyright and license details. */
-#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -35,8 +34,7 @@ main(int argc, char *argv[])
 		head(stdin, "<stdin>", n);
 	} else for(; argc > 0; argc--, argv++) {
 		if(!(fp = fopen(argv[0], "r"))) {
-			fprintf(stderr, "fopen %s: %s\n", argv[0],
-				strerror(errno));
+			weprintf("fopen %s:", argv[0]);
 			continue;
 		}
 		head(fp, argv[0], n);