Added upper bound on number of threads ZMap can use (#811)

* added --sender-threads upper bound

* updated manpages

* removed overriding user preference if they use too many sender threads and updated manpages

* Nitpicking Warning Message

* reverted changes on auto-generated files

---------

Co-authored-by: Zakir Durumeric <zakird@gmail.com>

src/zmap.c

@@ -1066,6 +1066,17 @@ int main(int argc, char *argv[])
 		    "too few targets relative to senders, dropping to one sender");
 		zconf.senders = 1;
 	}
+	// reserving 1 core for the receiver/monitor thread
+	int sender_cap = get_num_cores() - 1;
+	if (zconf.senders > sender_cap) {
+		log_warn(
+		    "zmap",
+		    "ZMap has been configured to use a larger number of sending threads (%d) than the number of "
+			"dedicated cores that can be assigned to sending packets. We advise using only "
+			"(number of cores - 1 = %d) sender threads such that every sender thread and the "
+			"one receiver thread each have a dedicated core. Using a large number of sender threads "
+			"will likely decrease performance, not increase it.", zconf.senders, sender_cap);
+	}
 #else
 	zconf.senders = args.sender_threads_arg;
 #endif