[ng] dist: refactor: reduce duplication in the 'dist-*' recipes

[ng] dist: refactor: reduce duplication in the 'dist-*' recipes

* lib/am/distcheck.mk: Here, with heavy refactorings, in particular
introducing ...
(am.dist.all-formats,
am.dist.all-targets,
am.dist.compress-cmd.gzip,
am.dist.uncompress-cmd.gzip,
am.dist.compress-cmd.bzip2,
am.dist.uncompress-cmd.bzip2,
am.dist.compress-cmd.lzip,
am.dist.uncompress-cmd.lzip
am.dist.compress-cmd.xz,
am.dist.uncompress-cmd.xz,
am.dist.create-cmd.zip,
am.dist.extract-cmd.zip): ... these new internal variables ...
(am.dist.create-archive-for-format.aux,
am.dist.create-archive-for-format,
am.dist.extract-archive-for-format.aux,
am.dist.extract-archive-for-format): ... and these new internal
make functions ...
(am.dist-gzip, am.dist-bzip2, am.dist-lzip, am.dist-xz,
am.dist.zip): ... and rewriting these rules as static-pattern
rules sharing the same recipe ...
(distcheck): ... and updating the recipe for this target.
* t/dist-formats.tap: Little adjustments for better usefulness
of verbose output when debugging.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>