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)