Before the interval expression was considered to be in seconds, now it is
just a number and the type of interval is considered.

- this changeset introduces also fix for bug#16432 (Events: error re interval
  misrepresents the facts)
  the code of event_timed::set_interval() was refactored anyway so it is meaningful to
  fix the bug in the same changeset.


include/my_time.h:
  - move enum interval_type to include/my_time.h so it can be used by functions
    in the whole server
sql/event.cc:
  - don't use second_part
  - fix small problem with create event xyz, when xyz exists -> make it error
    instead of warning if create_if_not is false.
sql/event.h:
  pass thd to mark_last_executed() to be able to call thd->end_time()
sql/event_executor.cc:
  - pass thd to event_timed::compute_next_execution_time()
  - a bit more DBUG info in the server log
  - handle error returned by event_timed::compute_next_execution_time()
sql/event_priv.h:
  - define the maximal possible value for interval_value
sql/event_timed.cc:
  - more docs
  - add static get_next_time() which sums a TIME with an interval
  - fix bug #16435 (Weekly events execute every second)
  Before the interval expression was considered to be in seconds, now it is
  just a number and the type of interval is considered.
  - fix for bug#16432 (Events: error re interval misrepresents the facts)
    (return an error if a value is too big or is negative - errmsg changed)
sql/item_timefunc.cc:
  - export get_interval_date()
  - refactor Item_date_add_interval::get_date() and extract the core
    to date_add_interval() in time.cc so it can be reused by the
    scheduler code in event_timed.cc
sql/item_timefunc.h:
  - export get_interval_value() so it can be reused in event_timed.cc in
    function static get_next_time()
  - move enum interval_type to include/my_time.h so it can be used by functions
    in the whole server
sql/mysql_priv.h:
  export the new function date_add_interval() added to time.cc
sql/share/errmsg.txt:
  - change error message to be appropriate as fix for bug#16432
    (Events: error re interval misrepresents the facts)
sql/sql_yacc.yy:
  - change error message to be appropriate as fix for bug#16432
    (Events: error re interval misrepresents the facts)
sql/time.cc:
  extract the core of Item_date_add_interval::get_date()
  to a function per Serg's request. The code can be reused
  to add und substract interval from a date.