Second version after review. Allow 'set autocommit' in procedures, but not
  functions or triggers. Can return error in run-time (when a function calls
  a procedure).


mysql-test/r/sp-error.result:
  New test case for BUG#12712.
mysql-test/t/sp-error.test:
  New test case for BUG#12712.
sql/set_var.cc:
  Made sys_autocommit external, to allow testing in sql_yacc.yy.
sql/set_var.h:
  Made sys_autocommit external, to allow testing in sql_yacc.yy.
sql/share/errmsg.txt:
  New error message for disallowing the setting of autocommit in stored functions and triggers.
sql/sp_head.h:
  New flag: has 'set autocommit', and testing for this in is_not_allowed_in_function().
sql/sql_yacc.yy:
  Disallow setting AUTOCOMMIT in stored function and triggers.