release: revamp rules to tag and upload the releases

release: revamp rules to tag and upload the releases

The older Makefile rules used to create and tag the releases were
based on an approach we now consider flawed: they over-mechanized
some delicate operations that are better performed manually *and*
double-checked by a developer, and at the same time they did not
run enough safety checks.

* Makefile.am (GIT, version_rx, stable_version_rx, beta_version_rx,
match_version, git_must_have_clean_workdir, determine_release_type):
New variables.
(git-release, git-dist): Remove, they are superseded by ...
(git-tag-release, git-upload-release): ... these new targets.