pgsql: Rename recently-added pg_stat_activity column from txn_start to

Log Message:
-----------
Rename recently-added pg_stat_activity column from txn_start to xact_start,
for consistency with other column names such as in pg_stat_database.

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        xact.c (r1.250 -> r1.251)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c?r1=1.250&r2=1.251)
    pgsql/src/backend/catalog:
        system_views.sql (r1.42 -> r1.43)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/system_views.sql?r1=1.42&r2=1.43)
    pgsql/src/backend/postmaster:
        pgstat.c (r1.162 -> r1.163)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c?r1=1.162&r2=1.163)
    pgsql/src/backend/utils/adt:
        pgstatfuncs.c (r1.43 -> r1.44)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/pgstatfuncs.c?r1=1.43&r2=1.44)
    pgsql/src/include/catalog:
        catversion.h (r1.423 -> r1.424)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.423&r2=1.424)
        pg_proc.h (r1.468 -> r1.469)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.468&r2=1.469)
    pgsql/src/include:
        pgstat.h (r1.64 -> r1.65)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pgstat.h?r1=1.64&r2=1.65)
    pgsql/src/test/regress/expected:
        rules.out (r1.132 -> r1.133)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/rules.out?r1=1.132&r2=1.133)