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)

mysql-test/r/parser.result

@@ -0,0 +1,68 @@
+DROP PROCEDURE IF EXISTS p26030;
+select "non terminated"$$
+non terminated
+non terminated
+select "terminated";$$
+terminated
+terminated
+select "non terminated, space"      $$
+non terminated, space
+non terminated, space
+select "terminated, space";      $$
+terminated, space
+terminated, space
+select "non terminated, comment" /* comment */$$
+non terminated, comment
+non terminated, comment
+select "terminated, comment"; /* comment */$$
+terminated, comment
+terminated, comment
+select "stmt 1";select "stmt 2 non terminated"$$
+stmt 1
+stmt 1
+stmt 2 non terminated
+stmt 2 non terminated
+select "stmt 1";select "stmt 2 terminated";$$
+stmt 1
+stmt 1
+stmt 2 terminated
+stmt 2 terminated
+select "stmt 1";select "stmt 2 non terminated, space"      $$
+stmt 1
+stmt 1
+stmt 2 non terminated, space
+stmt 2 non terminated, space
+select "stmt 1";select "stmt 2 terminated, space";      $$
+stmt 1
+stmt 1
+stmt 2 terminated, space
+stmt 2 terminated, space
+select "stmt 1";select "stmt 2 non terminated, comment" /* comment */$$
+stmt 1
+stmt 1
+stmt 2 non terminated, comment
+stmt 2 non terminated, comment
+select "stmt 1";select "stmt 2 terminated, comment"; /* comment */$$
+stmt 1
+stmt 1
+stmt 2 terminated, comment
+stmt 2 terminated, comment
+select "stmt 1";             select "space, stmt 2"$$
+stmt 1
+stmt 1
+space, stmt 2
+space, stmt 2
+select "stmt 1";/* comment */select "comment, stmt 2"$$
+stmt 1
+stmt 1
+comment, stmt 2
+comment, stmt 2
+DROP PROCEDURE IF EXISTS p26030; CREATE PROCEDURE p26030() BEGIN SELECT 1; END; CALL p26030()
+$$
+1
+1
+DROP PROCEDURE IF EXISTS p26030; CREATE PROCEDURE p26030() SELECT 1; CALL p26030()
+$$
+1
+1
+DROP PROCEDURE p26030;