pgsql: In 9.0 release notes, move libpq/ecpg up into their own sections,

Log Message:
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In 9.0 release notes, move libpq/ecpg up into their own sections, like I
did with server-side languages.

Modified Files:
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pgsql/doc/src/sgml:
release-9.0.sgml (r2.4 -> r2.5)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-9.0.sgml?r1=2.4&r2=2.5)