pgsql-server: Allow commas in BEGIN, START TRANSACTION, and SET

Log Message:
-----------
Allow commas in BEGIN, START TRANSACTION, and SET TRANSACTION, as required
by the SQL standard.  For backwards compatibility, however, continue to
accept the syntax without.  Minor editorialization in the reference pages
for these commands, too.

Modified Files:
--------------
    pgsql-server/doc/src/sgml/ref:
        begin.sgml (r1.32 -> r1.33)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/doc/src/sgml/ref/begin.sgml.diff?r1=1.32&r2=1.33)
        set_transaction.sgml (r1.19 -> r1.20)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/doc/src/sgml/ref/set_transaction.sgml.diff?r1=1.19&r2=1.20)
        start_transaction.sgml (r1.12 -> r1.13)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/doc/src/sgml/ref/start_transaction.sgml.diff?r1=1.12&r2=1.13)
    pgsql-server/src/backend/parser:
        gram.y (r2.470 -> r2.471)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/parser/gram.y.diff?r1=2.470&r2=2.471)
    pgsql-server/src/backend/tcop:
        utility.c (r1.224 -> r1.225)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/tcop/utility.c.diff?r1=1.224&r2=1.225)