Separate multi-words filenames by '_'

Rename foobar.ext to foo_bar.ext.

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

app/src/sys/win/command.c

@@ -2,7 +2,7 @@
 
 #include "config.h"
 #include "log.h"
-#include "strutil.h"
+#include "str_util.h"
 
 HANDLE cmd_execute(const char *path, const char *const argv[]) {
     STARTUPINFO si;