Bug#26030 (Parsing fails for stored routine w/multi-statement execution

enabled)

Before this fix, the lexer and parser would treat the ';' character as a
different token (either ';' or END_OF_INPUT), based on convoluted logic,
which failed in simple cases where a stored procedure is implemented as a
single statement, and used in a multi query.

With this fix:
- the character ';' is always parsed as a ';' token in the lexer,
- parsing multi queries is implemented in the parser, in the 'query:' rules,
- the value of thd->client_capabilities, which is the capabilities
  negotiated between the client and the server during bootstrap,
  is immutable and not arbitrarily modified during parsing (which was the
  root cause of the bug)

sql/sql_parse.cc

@@ -6169,6 +6169,11 @@ void mysql_parse(THD *thd, const char *inBuf, uint length,
               (thd->query_length= (ulong)(lip.found_semicolon - thd->query)))
             thd->query_length--;
           /* Actually execute the query */
+          if (*found_semicolon)
+          {
+            lex->safe_to_cache_query= 0;
+            thd->server_status|= SERVER_MORE_RESULTS_EXISTS;
+          }
           lex->set_trg_event_type_for_tables();
           mysql_execute_command(thd);
           query_cache_end_of_result(thd);