LibCore: Add File::OpenMode::DontCreate

Some applications may not want to have the ability to create a
file if it doesn't exist, but still be able to read and write
from it. The easy solution here would be just to not apply
O_CREAT when creating the flags, but to prevent breaking a ton
of applications, having a `DontCreate` mode is the best for now.

Userland/Libraries/LibCore/File.h

@@ -31,6 +31,7 @@ public:
         MustBeNew = 16,
         KeepOnExec = 32,
         Nonblocking = 64,
+        DontCreate = 128,
     };
 
     enum class ShouldCloseFileDescriptor {