floating point numbers

Some math functions did not check if the result is a valid number
(i.e. neither of +-inf or nan).

Fixed by validating the result where necessary and returning NULL in
case of invalid result.


BitKeeper/deleted/.del-matherr.c:
  Rename: sql/matherr.c -> BitKeeper/deleted/.del-matherr.c
configure.in:
  Removed DONT_USE_FINITE, it is not used anywhere.
include/my_global.h:
  isfinite() is a C99 macro which absoletes finite(). First try to use
  it, then fall back to finite() if the target platform has it,
  otherwise use our own implementation.
mysql-test/r/func_math.result:
  Added a test case for bug #31236.
mysql-test/r/strict.result:
  Fixed a test case which relied on old behavior.
mysql-test/t/func_math.test:
  Added a test case for bug #31236.
mysql-test/t/strict.test:
  Fixed a test case which relied on old behavior.
sql/field.cc:
  No need to check if the finite() or its equivalent is available.
sql/item_func.cc:
  Use fix_result() wherever the result can be one of +-inf or nan, 
  assuming the function arguments are valid numbers.
  Removed fix_result() from functions that are defined for all possible
  input numbers.
sql/item_func.h:
  Moved fix_result() from Item_dec_func to Item_func which is a common
  ancestor for Item_dec_func and Item_num_op.
sql/unireg.h:
  Removed POSTFIX_ERROR because no code returns it.