pgsql: Implement ALTER DATABASE SET TABLESPACE to move a whole database

Log Message:
-----------
Implement ALTER DATABASE SET TABLESPACE to move a whole database (or at least
as much of it as lives in its default tablespace) to a new tablespace.

Guillaume Lelarge, with some help from Bernd Helmle and Tom Lane

Modified Files:
--------------
    pgsql/doc/src/sgml/ref:
        alter_database.sgml (r1.20 -> r1.21)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/alter_database.sgml?r1=1.20&r2=1.21)
    pgsql/src/backend/commands:
        dbcommands.c (r1.215 -> r1.216)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c?r1=1.215&r2=1.216)
    pgsql/src/backend/parser:
        gram.y (r2.634 -> r2.635)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y?r1=2.634&r2=2.635)
    pgsql/src/backend/storage/lmgr:
        lmgr.c (r1.97 -> r1.98)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lmgr.c?r1=1.97&r2=1.98)
    pgsql/src/backend/tcop:
        utility.c (r1.300 -> r1.301)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c?r1=1.300&r2=1.301)
    pgsql/src/bin/psql:
        tab-complete.c (r1.173 -> r1.174)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/tab-complete.c?r1=1.173&r2=1.174)
    pgsql/src/include/commands:
        dbcommands.h (r1.47 -> r1.48)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/dbcommands.h?r1=1.47&r2=1.48)
    pgsql/src/include/storage:
        lmgr.h (r1.62 -> r1.63)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/lmgr.h?r1=1.62&r2=1.63)