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:
----
REL8_0_STABLE

Modified Files:
--------------
pgsql/doc/src/sgml:
Makefile (r1.75 -> r1.75.4.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/Makefile?r1=1.75&r2=1.75.4.1)
filelist.sgml (r1.41 -> r1.41.4.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/filelist.sgml?r1=1.41&r2=1.41.4.1)
release.sgml (r1.321.4.49 -> r1.321.4.50)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release.sgml?r1=1.321.4.49&r2=1.321.4.50)

Added Files:
-----------
pgsql/doc/src/sgml:
generate_history.pl (r1.1.8.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/generate_history.pl?rev=1.1.8.1&content-type=text/x-cvsweb-markup)
release-7.4.sgml (r1.1.8.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-7.4.sgml?rev=1.1.8.1&content-type=text/x-cvsweb-markup)
release-8.0.sgml (r1.1.8.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-8.0.sgml?rev=1.1.8.1&content-type=text/x-cvsweb-markup)
release-old.sgml (r1.1.8.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release-old.sgml?rev=1.1.8.1&content-type=text/x-cvsweb-markup)