Separate multi-words filenames by '_'

Rename foobar.ext to foo_bar.ext.

<https://github.com/Genymobile/scrcpy/pull/226#discussion_r209454865>

app/tests/test_strutil.c

@@ -1,7 +1,7 @@
 #include <assert.h>
 #include <string.h>
 
-#include "strutil.h"
+#include "str_util.h"
 
 static void test_xstrncpy_simple() {
     char s[] = "xxxxxxxxxx";