pgsql: Split the release notes into a separate file for each (active)

Log Message:
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Split the release notes into a separate file for each (active) major branch,
as per my recent proposal. release.sgml itself is now just a stub that should
change rarely; ideally, only once per major release to add a new include line.
Most editing work will occur in the release-N.N.sgml files. To update a back
branch for a minor release, just copy the appropriate release-N.N.sgml
file(s) into the back branch.

This commit doesn't change the end-product documentation at all, only the
source layout. However, it makes it easy to start omitting ancient information
from newer branches' documentation, should we ever decide to do that.

Tags:
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REL7_4_STABLE

Modified Files:
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pgsql/doc/src/sgml:
Makefile (r1.59.2.3 -> r1.59.2.4)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/Makefile?r1=1.59.2.3&r2=1.59.2.4)
filelist.sgml (r1.33 -> r1.33.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/filelist.sgml?r1=1.33&r2=1.33.2.1)
release.sgml (r1.235.2.67 -> r1.235.2.68)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release.sgml?r1=1.235.2.67&r2=1.235.2.68)

Added Files:
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pgsql/doc/src/sgml:
generate_history.pl (r1.1.10.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/generate_history.pl?rev=1.1.10.1&content-type=text/x-cvsweb-markup)
release-7.4.sgml (r1.1.10.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-7.4.sgml?rev=1.1.10.1&content-type=text/x-cvsweb-markup)
release-old.sgml (r1.1.10.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-old.sgml?rev=1.1.10.1&content-type=text/x-cvsweb-markup)