pgsql: Print proper cause of statement cancel, user interaction or

Log Message:
-----------
Print proper cause of statement cancel, user interaction or timeout.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        config.sgml (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.17&r2=1.18)
    pgsql/src/backend/storage/lmgr:
        proc.c (r1.163 -> r1.164)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/proc.c.diff?r1=1.163&r2=1.164)
    pgsql/src/backend/tcop:
        postgres.c (r1.459 -> r1.460)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.459&r2=1.460)
    pgsql/src/include/storage:
        proc.h (r1.81 -> r1.82)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/proc.h.diff?r1=1.81&r2=1.82)
    pgsql/src/test/regress/expected:
        prepared_xacts.out (r1.2 -> r1.3)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/prepared_xacts.out.diff?r1=1.2&r2=1.3)