Unify sending of call in S&P mode.

- Normally the key is handled by using 'sendspcall()' after <enter> key.
- F1 key in empty call field did also send the own call but by separate
  code. Now we use 'sendspcall()' in these case too.
Allow to use a custom S&P message isntead of the autogenerated one.

* extend message[] field by one
* Use the new SP_CALL_MSG if set
Allow to use a custom S&P message instead of the autogenerated one.

* extend message[] field by one
* add new Keyword S&P_CALL_MSG
* Use the new S&P_CALL_MSG if set
adapt :list command

* :list now shows also the S&P_CALL_MSG
* fix twisted definition of SP_TU_MSG and CQ_TU_MSG
* add #include guard
cleanup

* switch to using SP_TU_MSG and CQ_TU_MSG instead of hardcoded value
* fix 'parse_logcfg()'
Merge pull request #28 from dl1jbe/spcall

Send custom call in S&P mode