Get rid of err.h as it is not portable

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

components/num_files.c

@@ -1,6 +1,5 @@
 /* See LICENSE file for copyright and license details. */
 #include <dirent.h>
-#include <err.h>
 #include <stdio.h>
 #include <string.h>
 
@@ -14,7 +13,7 @@ num_files(const char *dir)
 	int num = 0;
 
 	if ((fd = opendir(dir)) == NULL) {
-		warn("Failed to get number of files in directory %s", dir);
+		fprintf(stderr, "Failed to get number of files in directory %s", dir);
 		return NULL;
 	}