Code-sign the Windows PuTTY binaries and installer.

Code-sign the Windows PuTTY binaries and installer.

Or, at least, potentially do so. The build script now has a slot into
which code-signing can be dropped by setting a variable in the bob
configuration to specify an appropriate command line.

The variable will typically need to point at a script wrapping the
actual signing tool, since there are lots of fiddly details
(timestamping countersignature, certificate, private key, etc) not
given on the command lines in this build script, on the basis that
they're local configuration questions for whoever is _running_ this
build script.