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.

Modified Files:
--------------
pgsql/doc/src/sgml:
Makefile (r1.114 -> r1.115)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/Makefile?r1=1.114&r2=1.115)
filelist.sgml (r1.60 -> r1.61)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/filelist.sgml?r1=1.60&r2=1.61)
release.sgml (r1.632 -> r1.633)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release.sgml?r1=1.632&r2=1.633)

Added Files:
-----------
pgsql/doc/src/sgml:
generate_history.pl (r1.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/generate_history.pl?rev=1.1&content-type=text/x-cvsweb-markup)
release-7.4.sgml (r1.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-7.4.sgml?rev=1.1&content-type=text/x-cvsweb-markup)
release-8.0.sgml (r1.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-8.0.sgml?rev=1.1&content-type=text/x-cvsweb-markup)
release-8.1.sgml (r1.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-8.1.sgml?rev=1.1&content-type=text/x-cvsweb-markup)
release-8.2.sgml (r1.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-8.2.sgml?rev=1.1&content-type=text/x-cvsweb-markup)
release-8.3.sgml (r1.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-8.3.sgml?rev=1.1&content-type=text/x-cvsweb-markup)
release-8.4.sgml (r1.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-8.4.sgml?rev=1.1&content-type=text/x-cvsweb-markup)
release-old.sgml (r1.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-old.sgml?rev=1.1&content-type=text/x-cvsweb-markup)