system/FileDescriptor: pass FileDescriptor to CheckDuplicate()

src/system/FileDescriptor.cxx

@@ -205,9 +205,9 @@ FileDescriptor::DisableCloseOnExec() noexcept
 }
 
 bool
-FileDescriptor::CheckDuplicate(int new_fd) noexcept
+FileDescriptor::CheckDuplicate(FileDescriptor new_fd) noexcept
 {
-	if (fd == new_fd) {
+	if (*this == new_fd) {
 		DisableCloseOnExec();
 		return true;
 	} else

src/system/FileDescriptor.hxx

@@ -168,8 +168,8 @@ public:
 	/**
 	 * Duplicate the file descriptor onto the given file descriptor.
 	 */
-	bool Duplicate(int new_fd) const noexcept {
-		return ::dup2(Get(), new_fd) == 0;
+	bool Duplicate(FileDescriptor new_fd) const noexcept {
+		return ::dup2(Get(), new_fd.Get()) == 0;
 	}
 
 	/**
@@ -178,7 +178,7 @@ public:
 	 * 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;
+	bool CheckDuplicate(FileDescriptor new_fd) noexcept;
 #endif
 
 #ifdef __linux__