pgsql: Improve CREATE/DROP/RENAME DATABASE so that when failing because

Log Message:
-----------
Improve CREATE/DROP/RENAME DATABASE so that when failing because the source
or target database is being accessed by other users, it tells you whether
the "other users" are live sessions or uncommitted prepared transactions.
(Indeed, it tells you exactly how many of each, but that's mostly just
because it was easy to do so.)  This should help forestall the gotcha of
not realizing that a prepared transaction is what's blocking the command.
Per discussion.

Modified Files:
--------------
    pgsql/src/backend/commands:
        dbcommands.c (r1.209 -> r1.210)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c?r1=1.209&r2=1.210)
    pgsql/src/backend/storage/ipc:
        procarray.c (r1.45 -> r1.46)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/procarray.c?r1=1.45&r2=1.46)
    pgsql/src/include/storage:
        procarray.h (r1.22 -> r1.23)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/procarray.h?r1=1.22&r2=1.23)