doc: Fix date in EPUB manifest
doc: Fix date in EPUB manifest

If there is no <date> element, the publication date for the EPUB
manifest is taken from the copyright year.  But something like
"1996-2013" is not a legal date specification.  So the EPUB output
currently fails epubcheck.

Put in a separate <date> element with the current year.  Put it in
legal.sgml, because copyright.pl already instructs to update that
manually, so it hopefully won't be missed.

xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,techdragon/Postgres-XL,zeroae/postgres-xl,lisakowen/gpdb,ovr/postgres-xl,adam8157/gpdb,jmcatamney/gpdb,adam8157/gpdb,ashwinstar/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,zeroae/postgres-xl,oberstet/postgres-xl,lisakowen/gpdb,pavanvd/postgres-xl,yazun/postgres-xl,pavanvd/postgres-xl,adam8157/gpdb,oberstet/postgres-xl,jmcatamney/gpdb,Postgres-XL/Postgres-XL,lisakowen/gpdb,adam8157/gpdb,ovr/postgres-xl,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,Postgres-XL/Postgres-XL,ashwinstar/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,jmcatamney/gpdb,pavanvd/postgres-xl,techdragon/Postgres-XL,jmcatamney/gpdb,xinzweb/gpdb,pavanvd/postgres-xl,oberstet/postgres-xl,jmcatamney/gpdb,ashwinstar/gpdb,zeroae/postgres-xl,Postgres-XL/Postgres-XL,Postgres-XL/Postgres-XL,lisakowen/gpdb,ovr/postgres-xl,lisakowen/gpdb,ovr/postgres-xl,yazun/postgres-xl,adam8157/gpdb,jmcatamney/gpdb,zeroae/postgres-xl,ashwinstar/gpdb,greenplum-db/gpdb,oberstet/postgres-xl,yazun/postgres-xl,50wu/gpdb,50wu/gpdb,jmcatamney/gpdb,Postgres-XL/Postgres-XL,ashwinstar/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,yazun/postgres-xl,ashwinstar/gpdb,techdragon/Postgres-XL,xinzweb/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,zeroae/postgres-xl,ashwinstar/gpdb,adam8157/gpdb,greenplum-db/gpdb,oberstet/postgres-xl,greenplum-db/gpdb,50wu/gpdb,ovr/postgres-xl,xinzweb/gpdb,techdragon/Postgres-XL,pavanvd/postgres-xl,techdragon/Postgres-XL,jmcatamney/gpdb,adam8157/gpdb,yazun/postgres-xl,adam8157/gpdb,greenplum-db/gpdb