pgsql: Disallow LOCK TABLE outside a transaction block (or function),

Log Message:
-----------
Disallow LOCK TABLE outside a transaction block (or function), since this case
almost certainly represents user error.  Per a gripe from Sebastian B�hm
and subsequent discussion.

Modified Files:
--------------
    pgsql/doc/src/sgml/ref:
        declare.sgml (r1.42 -> r1.43)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/declare.sgml?r1=1.42&r2=1.43)
        lock.sgml (r1.49 -> r1.50)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/lock.sgml?r1=1.49&r2=1.50)
    pgsql/src/backend/tcop:
        utility.c (r1.299 -> r1.300)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c?r1=1.299&r2=1.300)
    pgsql/src/test/regress/expected:
        privileges.out (r1.39 -> r1.40)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/privileges.out?r1=1.39&r2=1.40)
    pgsql/src/test/regress/sql:
        privileges.sql (r1.21 -> r1.22)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/privileges.sql?r1=1.21&r2=1.22)