tuned signature: more operations (following Isabelle/Scala);

--- a/src/Pure/System/process_result.ML	Wed Feb 05 22:54:47 2025 +0100
+++ b/src/Pure/System/process_result.ML	Wed Feb 05 23:11:07 2025 +0100
@@ -6,6 +6,7 @@
 
 signature PROCESS_RESULT =
 sig
+  val startup_failure_rc: int
   val interrupt_rc: int
   val timeout_rc: int
   type T
@@ -29,6 +30,7 @@
 structure Process_Result: PROCESS_RESULT =
 struct
 
+val startup_failure_rc = 127
 val interrupt_rc = 130
 val timeout_rc = 142