Keep track on the default value and use it. (Or NULL, if not declared.)


mysql-test/r/sp.result:
  New testcases for BUG#2776.
mysql-test/t/sp.test:
  New testcases for BUG#2776.
sql/sp_pcontext.cc:
  Initialize local variable default value.
sql/sp_pcontext.h:
  New method for saving default value.
sql/sql_yacc.yy:
  If DEFAULT is use as a value in SET variable = ... in an SP, actually use the default.