- Honor unsigned_flag in the corresponding functions
- Use compare_int_signed_unsigned()/compare_int_unsigned_signed() instead of explicit comparison in GREATEST() and LEAST()


mysql-test/r/case.result:
  Added test case for bug #20924
mysql-test/r/func_if.result:
  Added test case for bug #20924
mysql-test/r/func_test.result:
  Added test case for bug #20924
mysql-test/r/user_var.result:
  Added test case for bug #20924
mysql-test/t/case.test:
  Added test case for bug #20924
mysql-test/t/func_if.test:
  Added test case for bug #20924
mysql-test/t/func_test.test:
  Added test case for bug #20924
mysql-test/t/user_var.test:
  Added test case for bug #20924
sql/item_cmpfunc.cc:
  Bug #20924: CAST(expr as UNSIGNED) returns SIGNED value when used in various functions
  
  - Moved some code out of Arg_comparator to external functions to be reused in Item_func_min_max
  - Fixed IFNULL(), IF(), CASE() and COALESCE()
sql/item_cmpfunc.h:
  Bug #20924: CAST(expr as UNSIGNED) returns SIGNED value when used in various functions
  
  - Moved some code out of Arg_comparator to external functions to be reused in Item_func_min_max
sql/item_func.cc:
  Bug #20924: CAST(expr as UNSIGNED) returns SIGNED value when used in various functions
  
  Fixed LEAST(), GREATEST() and "SET @A=..." parts
sql/item_func.h:
  Bug #20924: CAST(expr as UNSIGNED) returns SIGNED value when used in various functions
  
  Fixed "SET @A=..." part
sql/sql_class.h:
  Bug #20924: CAST(expr as UNSIGNED) returns SIGNED value when used in various functions
  
  Fixed "SET @A=..." part