NAME_CONST('whatever', -1) * MAX(whatever) bombed since -1 was
not seen as constant, but as FUNCTION_UNARY_MINUS(constant)
while we are at the same time pretending it was a basic const
item. This confused the aggregate handlers in exciting ways.
We now make NAME_CONST() behave more consistently.


mysql-test/r/func_misc.result:
  show that a combination of NAME_CONST('x', -y) and an aggregate
  no longer crashes the server.
mysql-test/t/func_misc.test:
  show that a combination of NAME_CONST('x', -y) and an aggregate
  no longer crashes the server.
sql/ha_ndbcluster_cond.cc:
  tell cluster about "new" function type NEG_FUNC.
  (this was previous identified as UNKNOWN_FUNC,
  so we just handle it the same way, that's all.)
sql/ha_ndbcluster_cond.h:
  tell cluster about "new" function type NEG_FUNC.
  (this was previous identified as UNKNOWN_FUNC,
  so we just handle it the same way, that's all.)
sql/item.cc:
  make NAME_CONST() transparent in that type() of
  -constant is that of constant, not that of unary
  minus (id est, FUNC_ITEM).
sql/item.h:
  Move constructor to item.cc
sql/item_func.h:
  Revert Bug#30832; we can apply the magic more narrowly
  (just for NAME_CONST() rather than all Item_func_neg).
  
  Introduce new function type "NEG_FUNC."