Fix incorrect use of variable arguments in run_parts() and simplify code

The run_parts() implementation used variable arguments in a bad way,
resulting in it not working properly.  Replaced va_*() code with a
simple cmd argument that's passed to each script called.

Also, the opendir()/readdir() code has been replaced with scandir()
using regular alphasort(). This simplifies the code somewhat as well.

Otherwise, minor cleanups.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

plugins/resolvconf.c

@@ -59,7 +59,7 @@ static void setup(void *UNUSED(arg))
 	touch("/etc/resolvconf/run/enable-updates");
 
 	chdir("/etc/resolvconf/run/interface");
-	run_parts("/etc/resolvconf/update.d", "-i", NULL);
+	run_parts("/etc/resolvconf/update.d", "-i");
 	chdir("/");
 #endif /* USE_ETC_RESOLVCONF_RUN */
 }