Get rid of err.h as it is not portable

Replace warn() and warnx() with fprintf() and add <stdio.h> where
necessary.

components/hostname.c

@@ -1,5 +1,5 @@
 /* See LICENSE file for copyright and license details. */
-#include <err.h>
+#include <stdio.h>
 #include <unistd.h>
 
 #include "../util.h"
@@ -8,7 +8,7 @@ const char *
 hostname(void)
 {
 	if (gethostname(buf, sizeof(buf)) == -1) {
-		warn("hostname");
+		fprintf(stderr, "gethostbyname failed");
 		return NULL;
 	}