Allow the gpg utility to be overridden on the command line. For
example, './tool/depot/publish GPG="ssh 10.0.1.3 gpg" ...' would invoke
a remote gpg over SSH.
Fix#2981
The timestamp-based rules of make do not work well for the publish tool
because depot archives are often re-created with the same content but a
different modification time, in particular when creating archives via
'FORCE'. This way, those archives are re-published every time, which
becomes a time-consuming operation since all archives must be signed.
This patch filters the targets based on the content that is already
present in the public/ location. All existing archives are skipped.
This patch replaces the toy downloader (that merely copied files
locally, for testing) with the mechanics needed to download files from a
www server.
It also changes the use of GPG to use detached signatures.
Issue #2339
