generic makefile: also delete deb_dist folder when running make deb-clean
generic makefile:

echo errors to stderr

implemented new make targets:

make git-tag-sign
   git tag (-s) sign latest pkg_version_with_revision from debian/changelog. Only a repository sanity check. Not for security purposes!
make git-tag-verify
   git tag (-v) verify latest pkg_version_with_revision from debian/changelog. Only a repository sanity check. Not for security purposes!
make git-tag-check
   Check if current git head is a signed git tag. Only a repository sanity check. Not for security purposes!
make git-commit-verify
   Check if current git head is a signed git commit. Only a repository sanity check. Not for security purposes!
make git-verify
   Combination of tag-check and commit-verify. Only a repository sanity check. Not for security purposes!

Implements: https://phabricator.whonix.org/T1