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

Log Message:
-----------
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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REL8_2_STABLE

Modified Files:
--------------
pgsql/doc/src/sgml:
Makefile (r1.81.2.3 -> r1.81.2.4)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/Makefile?r1=1.81.2.3&r2=1.81.2.4)
filelist.sgml (r1.49 -> r1.49.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/filelist.sgml?r1=1.49&r2=1.49.2.1)
release.sgml (r1.488.2.26 -> r1.488.2.27)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release.sgml?r1=1.488.2.26&r2=1.488.2.27)

Added Files:
-----------
pgsql/doc/src/sgml:
generate_history.pl (r1.1.4.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/generate_history.pl?rev=1.1.4.1&content-type=text/x-cvsweb-markup)
release-7.4.sgml (r1.1.4.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-7.4.sgml?rev=1.1.4.1&content-type=text/x-cvsweb-markup)
release-8.0.sgml (r1.1.4.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-8.0.sgml?rev=1.1.4.1&content-type=text/x-cvsweb-markup)
release-8.1.sgml (r1.1.4.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-8.1.sgml?rev=1.1.4.1&content-type=text/x-cvsweb-markup)
release-8.2.sgml (r1.1.4.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-8.2.sgml?rev=1.1.4.1&content-type=text/x-cvsweb-markup)
release-old.sgml (r1.1.4.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-old.sgml?rev=1.1.4.1&content-type=text/x-cvsweb-markup)