pgsql-server: Add WAL logging for CREATE/DROP DATABASE and CREATE/DROP

Log Message:
-----------
Add WAL logging for CREATE/DROP DATABASE and CREATE/DROP TABLESPACE.
Fix TablespaceCreateDbspace() to be able to create a dummy directory
in place of a dropped tablespace's symlink.  This eliminates the open
problem of a PANIC during WAL replay when a replayed action attempts
to touch a file in a since-deleted tablespace.  It also makes for a
significant improvement in the usability of PITR replay.

Modified Files:
--------------
    pgsql-server/doc/src/sgml:
        backup.sgml (r2.46 -> r2.47)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/doc/src/sgml/backup.sgml.diff?r1=2.46&r2=2.47)
    pgsql-server/src/backend/access/transam:
        rmgr.c (r1.15 -> r1.16)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/rmgr.c.diff?r1=1.15&r2=1.16)
    pgsql-server/src/backend/commands:
        dbcommands.c (r1.141 -> r1.142)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/commands/dbcommands.c.diff?r1=1.141&r2=1.142)
        tablespace.c (r1.9 -> r1.10)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/commands/tablespace.c.diff?r1=1.9&r2=1.10)
    pgsql-server/src/include/access:
        rmgr.h (r1.12 -> r1.13)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/access/rmgr.h.diff?r1=1.12&r2=1.13)
    pgsql-server/src/include/commands:
        dbcommands.h (r1.33 -> r1.34)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/commands/dbcommands.h.diff?r1=1.33&r2=1.34)
        tablespace.h (r1.3 -> r1.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/commands/tablespace.h.diff?r1=1.3&r2=1.4)
    pgsql-server/src/include:
        port.h (r1.56 -> r1.57)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/port.h.diff?r1=1.56&r2=1.57)
    pgsql-server/src/port:
        path.c (r1.33 -> r1.34)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/port/path.c.diff?r1=1.33&r2=1.34)