Makefile: Simplify the release targets.

Previously, we had a separate release-upload target that a user might
mistake as something useful to call directly, (which would have the
undesired effect or uploading a new package, but without first making
all the checks that we want).

So we eliminate that target, (folding its actions into "make
release"), and we also rename the several release-verify-foo targets
to simply verify-foo. This leaves as the only targets with "release"
in the name as "release" and "release-message". Both of these are
intended for the user to call directly.