tool_operate: cleanups

- move the state struct from config to global. It is used as a single
  instance anyway so might as well be a single one to save memory.
- simplify and combine several conditions
- set default retry delay inititally
- use better struct field names to make it easier to understand their
  purposes
- remove the state->outfiles field as it was not necessary
- remove superfluous glob cleanup call
- move conditions around to remove an indent level
- move the ->url NULL check

Takes single_transfer()'s complexity score down from 78 to 68.

Closes #18226

src/tool_operate.h

@@ -80,7 +80,7 @@ struct per_transfer {
 };
 
 CURLcode operate(int argc, argv_item_t argv[]);
-void single_transfer_cleanup(struct OperationConfig *config);
+void single_transfer_cleanup(void);
 
 extern struct per_transfer *transfers; /* first node */