First step


sql/item.h:
  Add function to check for partition function whether allowed or not.
  In this first step allow only a field on its own.
  Further steps will add a lot more possibilities.
sql/partition_info.cc:
  Add a check whether the partition function is allowed by walking the
  partition function expression.
sql/share/errmsg.txt:
  Add a new error code for partition function not allowed