Get rid of err.h as it is not portable

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

components/run_command.c

@@ -1,5 +1,4 @@
 /* See LICENSE file for copyright and license details. */
-#include <err.h>
 #include <stdio.h>
 #include <string.h>
 
@@ -13,7 +12,7 @@ run_command(const char *cmd)
 
 	fp = popen(cmd, "r");
 	if (fp == NULL) {
-		warn("Failed to get command output for %s", cmd);
+		fprintf(stderr, "Failed to get command output for %s", cmd);
 		return NULL;
 	}
 	p = fgets(buf, sizeof(buf) - 1, fp);