Fix for bug #24912 "problems with bigint in abs() ceiling() round() truncate() mod()" and a number of related problems:

- unsigned flag was not handled correctly for a number of mathematical funcions, which led to incorrect results
- passing large values as the number of decimals to ROUND() resulted in incorrect results and even server crashes in some cases
- reverted the fix and the testcase for bug #10083 as it violates the manual
- fixed some testcases which relied on broken ROUND() behavior


mysql-test/r/func_math.result:
  - Removed the testcase for bug #10083 (not a bug according to the manual)
  - Changed the testcase for bug #9837 to expect a correct ROUND() behavior
  - Added testcases for bug #24912 and all related bugs found
mysql-test/r/type_newdecimal.result:
  Fixed a truncate() testcase which relied on broken behavior
mysql-test/t/func_math.test:
  - Removed the testcase for bug #10083 (not a bug according to the manual)
  - Changed the testcase for bug #9837 to expect a correct ROUND() behavior
  - Added testcases for bug #24912 and all related bugs found
sql/item_func.cc:
  Various changes to fix bug #24912 and all related bugs found:
  - honor unsigned_flag in various Item_* functions
  - correctly handle out-of-range numbers of decimals in Item_func_round::fix_length_and_dec()
  - changed the argument specifying the number of decimals in my_double_round() from int to longlong, added a new argument to pass the 'unsigned flag'
  - changed my_double_round() to correctly handle large values passed as the 'number of decimals' argument
  - added a my_double_round() analog for BIGINT UNSIGNED arguments (my_unsigned_round())
  - fixed Item_func_round()::int_op() to not overflow even when the result is within integer range
  - fixed a bug Item_founc_round()::decimal_op() which resulted in crash when a large number of decimals was passed to my_decimal_round()
sql/item_func.h:
  Various fixed to correctly handle unsigned values.
sql/item_strfunc.cc:
  Changed the call to my_double_round() to match the new declaration.
sql/mysql_priv.h:
  Changed the declaration for my_double_round() to be able pass arbitrary integers as number of decimals (both signed and unsigned)