Added finer scope control for default clauses of local variable
  declarations.


mysql-test/r/sp.result:
  New test case for BUG#14376.
mysql-test/t/sp.test:
  New test case for BUG#14376.
sql/sp_pcontext.cc:
  Added boundary variable for local parameters/variables,
  for better scope control of default values.
sql/sp_pcontext.h:
  Added boundary variable for local parameters/variables,
  for better scope control of default values.
sql/sql_yacc.yy:
  Make the variables of the current DECLARE "invisible" to its DEFAULT clause.