tool_operate: remove call to abort()

Make an assert and return a plain error instead. No abort in release
code.

- also remove from two examples
- add as a banned function in checksrc
- document the banned state

Closes #22182

docs/examples/multi-event.c

@@ -205,7 +205,7 @@ static int handle_socket(CURL *curl, curl_socket_t s, int action, void *userp,
     }
     break;
   default:
-    abort();
+    return -1; /* unknown */
   }
 
   return 0;

docs/examples/multi-uv.c

@@ -219,7 +219,7 @@ static int cb_socket(CURL *curl, curl_socket_t s, int action,
     }
     break;
   default:
-    abort();
+    return -1; /* unknown */
   }
 
   return 0;