c/fields: Warn on some broken alioth paths in Vcs-Cvs and Vcs-Git

Involves a slight generalization of VCS_CANONIFY to allow custom tags.

Also includes tests.

Closes: #652595

[nthykier: minor tag description changes]

Signed-off-by: Samuel Bronson <naesten@gmail.com>
Signed-off-by: Niels Thykier <niels@thykier.net>