Format error messages properly

Make use of strerror(errno) and format all errors equally:
function ['parameters']: error message

components/hostname.c

@@ -1,5 +1,7 @@
 /* See LICENSE file for copyright and license details. */
+#include <errno.h>
 #include <stdio.h>
+#include <string.h>
 #include <unistd.h>
 
 #include "../util.h"
@@ -8,7 +10,7 @@ const char *
 hostname(void)
 {
 	if (gethostname(buf, sizeof(buf)) == -1) {
-		fprintf(stderr, "gethostbyname failed");
+		fprintf(stderr, "gethostbyname: %s\n", strerror(errno));
 		return NULL;
 	}