core/execute-serialize: clean up stdio serialization

* Do not interleave root_directory_as_fd with stdio fields
* Do not use different serialization key for different modes
  pointing to same path
* Escape stdio file paths (as per
  9be46b1da8)

test/fuzz/fuzz-execute-serialize/initial

@@ -63,14 +63,10 @@ exec-context-stdio-as-fds=
 exec-context-std-input-fd-name=
 exec-context-std-output-fd-name=
 exec-context-std-error-fd-name=
+exec-context-std-input-data=
 exec-context-std-input-file=
 exec-context-std-output-file=
-exec-context-std-output-file-append=
-exec-context-std-output-file-truncate=
 exec-context-std-error-file=
-exec-context-std-error-file-append=
-exec-context-std-error-file-truncate=
-exec-context-stdin-data=
 exec-context-tty-path=
 exec-context-tty-reset=
 exec-context-tty-vhangup=