Send custom call in S&P mode

Pull request contains changes to allow a user to create its own call message for S&P mode.

You can define the message with the new Keyword 'S&P_CALL_MSG=...'. If the message is undefined the old automatic mode (see sendspcall()) is used.

The new message is useful if you
- want to embed special characters like + or - in your call (as Sebastian Kricner suggested) or
- if you want to add hiscall to the message or
- if you want to send your call more than once.

It makes the old hardcoded logic unneccesary but keeps them for the moment.