Libpwmd fixes for --idle.

When using the --idle option to fetchmail along with pwmd, the connection to the pwmd server would be kept open. So close the connection after the first server with an --idle option to prevent looking up pwmd credentials for other mail servers.

Also reopen the connection if any pwmd socket options have changed since the previous poll. Before, only the data file and socket location were tested for changes.

And there is no need to lock the data file mutex for fetchmail connections. Locking is usually reserved for write operations to the data file.