subprocess: avoid enum in subprocess_manager_set

Change prototype of subprocess_manager_set() to avoid using an enum
as the argument passed to va_start(), as this causes warnings on newer
compilers which complain about

      passing an object that undergoes default argument promotion
      to 'va_start' has undefined behavior [-Werror,-Wvarargs]

The enum undergoes default promotion, and thus it may cause undefined
behavior. Therefore, change the enum argument to and integer to avoid
this warning.

src/common/libsubprocess/subprocess.c

@@ -1082,7 +1082,7 @@ subprocess_manager_reap_all (struct subprocess_manager *sm)
 }
 
 int
-subprocess_manager_set (struct subprocess_manager *sm, sm_item_t item, ...)
+subprocess_manager_set (struct subprocess_manager *sm, int item, ...)
 {
     va_list ap;
 

src/common/libsubprocess/subprocess.h

@@ -66,8 +66,7 @@ struct subprocess_manager * subprocess_manager_create (void);
 /*
  *  Set value for item [item] in subprocess manager [sm]
  */
-int subprocess_manager_set (struct subprocess_manager *sm,
-	sm_item_t item, ...);
+int subprocess_manager_set (struct subprocess_manager *sm, int item, ...);
 
 /*
  *  Free memory associated with a subprocess manager object: