pgsql: Replace "slave" to "standby" in documentation for consistent

Log Message:
-----------
Replace "slave" to "standby" in documentation for consistent terminology.
Almost all of the terms in docs and messages were replaced, but still
remains in a few comments and README files in codes.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        backup.sgml (r2.155 -> r2.156)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/backup.sgml?r1=2.155&r2=2.156)
        dblink.sgml (r1.11 -> r1.12)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/dblink.sgml?r1=1.11&r2=1.12)
        external-projects.sgml (r1.19 -> r1.20)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/external-projects.sgml?r1=1.19&r2=1.20)
        high-availability.sgml (r1.70 -> r1.71)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/high-availability.sgml?r1=1.70&r2=1.71)
        protocol.sgml (r1.88 -> r1.89)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/protocol.sgml?r1=1.88&r2=1.89)
        release-9.0.sgml (r2.27 -> r2.28)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-9.0.sgml?r1=2.27&r2=2.28)
        release-alpha.sgml (r2.1 -> r2.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-alpha.sgml?r1=2.1&r2=2.2)