Further release automation.

Further release automation.

I've added extra modes to release.pl which should automate the more
tedious parts of the deployment phase: uploading the release build to
all the places it needs to go, checking its integrity once it gets
there, verifying that everything can be downloaded again usefully,
checking content-types etc.

The new version should check more thoroughly (it checks the whole FTP
and HTTP download directories, so it will spot errors like failing to
update the FTP 'latest' symlink), and take fewer commands to run.