merge and additional clarifications

mysql-test/r/func_sapdb.result:
  Update test for 42661/42662 to use both TIME(...) and CAST(... AS TIME).
  They go through the same code-path here, but may not in the future, and
  besides, it's clearer this way.
mysql-test/t/func_sapdb.test:
  Update test for 42661/42662 to use both TIME(...) and CAST(... AS TIME).
  They go through the same code-path here, but may not in the future, and
  besides, it's clearer this way.
sql/item_timefunc.cc:
  - fix SEC_TO_TIME(...) for signed values
  - fix TIME(...) and CAST(... AS TIME) for signed values