Kernel: Expose blocking and cloexec fd flags in ProcFS

Author: https://github.com/bugaevc
Commit: 9a41dda029
Pull-request: https://github.com/SerenityOS/serenity/pull/611

Kernel/Process.h

@@ -95,6 +95,7 @@ public:
 
     FileDescription* file_description(int fd);
     const FileDescription* file_description(int fd) const;
+    int fd_flags(int fd) const;
 
     template<typename Callback>
     static void for_each(Callback);