Phase 3: Made qualified names work for functions as well.


mysql-test/r/sp-security.result:
  New testcases for functions with qualified names.
mysql-test/t/sp-security.test:
  New testcases for functions with qualified names.
sql/item_func.cc:
  Added error handling for stored function, if it doesn't exist.
sql/item_func.h:
  Set null_value if execution of a stored function fails.
sql/mysql_priv.h:
  Reverted previous change: No optional args for mysql_change_db().
  (SPs use a specially tailored function instead.)
sql/sp.cc:
  Copied mysql_change_db() from sql_db.cc and modified specially for SPs.
sql/sp_head.cc:
  Fixed error handling for errors in functions during query/statement execution.
sql/sql_db.cc:
  Reverted previous change: No optional args for mysql_change_db().
  (SPs use a specially tailored function instead.)
sql/sql_yacc.yy:
  Reworked the stored function/UDF invokation parsing and added qualified names
  for stored functions. UDFs now have precedence over stored functions (whith
  unqualified name). When using an unqualified name, only IDENT_sys is allowed
  (i.e. no unreserved keywords), since we get unresolvable reduce/reduce conflicts
  otherwise.