system/FileDescriptor: add CheckDuplicate()

src/system/FileDescriptor.hxx

@@ -146,6 +146,14 @@ public:
 	bool Duplicate(int new_fd) const noexcept {
 		return ::dup2(Get(), new_fd) == 0;
 	}
+
+	/**
+	 * Similar to Duplicate(), but if destination and source file
+	 * descriptor are equal, clear the close-on-exec flag.  Use
+	 * this method to inject file descriptors into a new child
+	 * process, to be used by a newly executed program.
+	 */
+	bool CheckDuplicate(int new_fd) noexcept;
 #endif
 
 #ifdef USE_EVENTFD