pgsql: Add reference to libpq environment variables for client

Log Message:
-----------
Add reference to libpq environment variables for client applications.

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
clusterdb.sgml (r1.16 -> r1.17)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/clusterdb.sgml.diff?r1=1.16&r2=1.17)
createdb.sgml (r1.40 -> r1.41)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/createdb.sgml.diff?r1=1.40&r2=1.41)
createlang.sgml (r1.33 -> r1.34)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/createlang.sgml.diff?r1=1.33&r2=1.34)
createuser.sgml (r1.40 -> r1.41)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/createuser.sgml.diff?r1=1.40&r2=1.41)
dropdb.sgml (r1.26 -> r1.27)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/dropdb.sgml.diff?r1=1.26&r2=1.27)
droplang.sgml (r1.25 -> r1.26)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/droplang.sgml.diff?r1=1.25&r2=1.26)
dropuser.sgml (r1.31 -> r1.32)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/dropuser.sgml.diff?r1=1.31&r2=1.32)
pg_dump.sgml (r1.76 -> r1.77)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/pg_dump.sgml.diff?r1=1.76&r2=1.77)
pg_dumpall.sgml (r1.48 -> r1.49)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/pg_dumpall.sgml.diff?r1=1.48&r2=1.49)
pg_restore.sgml (r1.50 -> r1.51)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/pg_restore.sgml.diff?r1=1.50&r2=1.51)
psql-ref.sgml (r1.135 -> r1.136)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/psql-ref.sgml.diff?r1=1.135&r2=1.136)
vacuumdb.sgml (r1.35 -> r1.36)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/vacuumdb.sgml.diff?r1=1.35&r2=1.36)