We pretended that TIMEDIFF() would always return positive results;
this gave strange results in comparisons of the TIMEDIFF(low,hi)<TIME(0)
type that rendered a negative result, but still gave false in comparison.
We also inadvertantly dropped the sign when converting times to
decimal.

CAST(time AS DECIMAL) handles signs of the times correctly.
TIMEDIFF() marked up as signed. Time/date comparison code switched to
signed for clarity.

mysql-test/r/func_sapdb.result:
  show that time-related comparisons work with negative
  time values now.
  show that converting time to DECIMAL no longer drops sign.
mysql-test/t/func_sapdb.test:
  show that time-related comparisons work with negative
  time values now.
  show that converting time to DECIMAL no longer drops sign.
sql/item_cmpfunc.cc:
  signed returns
sql/item_cmpfunc.h:
  signed now (time/date < > =)
sql/item_func.cc:
  signed now
sql/item_timefunc.h:
  Functions such as TIMEDIFF() return signed results!
  The file-comments pretended we were doing that all along, anyway...
sql/my_decimal.cc:
  heed sign when converting time to my_decimal;
  times may actually be negative!
  Needed for SELECT CAST(time('-73:42:12') AS DECIMAL);
sql/mysql_priv.h:
  using signed for dates and times now