let update-xmlsectool merge to master with message

When update-xmlsectool does its work in the vendor branch, it needs
to merge the changes back into master. Previously, this last step had
a manual intervetion, requiring review/change of the merge message.
We now do this as part of the script.
make date calcs portable in build-metadata

build-metadata previously used the non-portable -d option to
Gnu date(1). This didn't work on non-gnu systems like Solaris and
Darwin (Mac OS X). Changed to use perl for the same calcluation.
Should work everywhere.