Add truncate: to StandardOutput= etc.

This adds the ability to specify truncate:PATH for StandardOutput= and
StandardError=, similar to the existing append:PATH. The code is mostly
copied from the related append: code. Fixes #8983.

src/core/execute.h

@@ -61,6 +61,7 @@ typedef enum ExecOutput {
         EXEC_OUTPUT_NAMED_FD,
         EXEC_OUTPUT_FILE,
         EXEC_OUTPUT_FILE_APPEND,
+        EXEC_OUTPUT_FILE_TRUNCATE,
         _EXEC_OUTPUT_MAX,
         _EXEC_OUTPUT_INVALID = -1
 } ExecOutput;