FTP: move FTP-specific struct field to ftpc_conn

"wait_data_conn" was added to the connectionbits in commit c834213ad5 for
handling active FTP connections but as it is purely FTP specific and now
only ever accessed by ftp.c I moved it into the FTP connection struct.

lib/ftp.h

@@ -139,6 +139,7 @@ struct ftp_conn {
                        already has been done */
   bool cwdfail;     /* set TRUE if a CWD command fails, as then we must prevent
                        caching the current directory */
+  bool wait_data_conn; /* this is set TRUE if data connection is waited */
   char *prevpath;   /* conn->path from the previous transfer */
   char transfertype; /* set by ftp_transfertype for use by Curl_client_write()a
                         and others (A/I or zero) */