Additional improvements aimed at improving operational
reliability of the SST scripts:

1) Script need to give rsync and stunnel a short time to
   terminate after "kill -9" before the first PID check
   using ps utility;
2) The temporary file used to create the binlog index could
   sometimes remain in the data directory if tar failed and
   then may be reused without being cleaned up (the next
   time when SST was run) - now it's fixed;
3) The temporary file used to build the binlog index is now
   created using mktemp and, if this variable is present in
   the configuration file, in tmpdir;
4) Checking the secret tag in SST via rsync is made faster
   and does not require creating a temporary file, which
   could remain in the data directory in case of failure;
5) Added "-F" option to grep to check the tag when using
   mariabackup/xtrabackup-v2 - to avoid possible collisions
   in case of special characters in the tag value (unlikely
   scenario, but the new check is more reliable).