Replaced the dumb in-handler/not-in-handler check with a proper recursion
    check of handlers being executed.
    (Re-commit in a different tree, to make push possible.)


mysql-test/r/sp.result:
  New test case for BUG#6127.
mysql-test/t/sp.test:
  New test case for BUG#6127.
sql/sp_head.cc:
  Replaced the setting of ctx->in_handler with a enter/exit handler methods.
sql/sp_rcontext.cc:
  Replaced the boolean in_handler flag with a stack of handlers being exectuted, for proper recursion check.
sql/sp_rcontext.h:
  Replaced the boolean in_handler flag with a stack of handlers being exectuted, for proper recursion check.
  (And added some comments in the sp_rcontext class.)