The second cleanup patch in scope of BUG#11986.

1. Introduce parse_sql() as a high-level replacement for MYSQLparse().
parse_sql() is responsible to switch and restore "parser context"
(THD::m_lip for now).

2. Fix typo in sp.cc: THD::spcont should be reset *before* calling
the parser.

sql/sql_class.cc

@@ -342,7 +342,8 @@ THD::THD()
    in_lock_tables(0),
    bootstrap(0),
    derived_tables_processing(FALSE),
-   spcont(NULL)
+   spcont(NULL),
+   m_lip(NULL)
 {
   ulong tmp;