pgsql-server: Fix some shortcomings in psql's autocommit-off mode

Log Message:
-----------
Fix some shortcomings in psql's autocommit-off mode concerning detection
of commands for which a transaction block should not be forced.  Recognize
VACUUM and other PreventTransactionChain commands; handle nested /* .. */
comments correctly; handle multibyte encodings correctly.
Michael Paesold with some kibitzing from Tom Lane.

Modified Files:
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    pgsql-server/doc/src/sgml/ref:
        psql-ref.sgml (r1.121 -> r1.122)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/doc/src/sgml/ref/psql-ref.sgml.diff?r1=1.121&r2=1.122)
    pgsql-server/src/bin/psql:
        common.c (r1.90 -> r1.91)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/bin/psql/common.c.diff?r1=1.90&r2=1.91)