Udocker

* Fix 2 unit tests

* Image list does not truncate long names: closes 349
* Fix conditional warning in verify image
* Fix and improve udocker high level tests

* Fix missing f (format) for string
* Fix bugs with dict .items()
* Solving several pylint issues
* Remove use2to3: closes 358

* Add --entrypoint to run --help
* Set docker hub registry registry-1.docker.io
* Fix repository name in search --list-tags
* Improve tests: udocker_test.sh and udocker_test-run.sh
* Documentation revision and improvements
* Add licenses and licenses notice to documentation
* Add test instructions
* Issues with --allow-root in Python 3.8
* Add security policy SECURITY.md
* Remove old Python 2 tests
* Fix configuration hierarchy, configuration files
* Update documentation: README, user and install manuals
* Fix sqa and config

* Prepare to move the stable code for Python 3 and Python 2 >= 2.6 to master
* Installation procedure changed since 1.1.x series see the `installation_manual`
* Improve user and installation documentation
* Extract documentation upon installation
* Add codemeta.json, metadata for the software
* Add support for `faccessat2()` in Pn and Fn execution modes
* Fix support for `newfstatat()` in Pn execution modes
* Add Fn libraries for Fedora 34 and Ubuntu 21.04
* Remove broken links in FileUtil.remove()
* Update minimum udocker tools tarball to 1.2.8
* Cmd and entrypoint metadata and arguments processing changed to mimic docker
* Improve removal of files and links in install and filebind restore
* Add follow location option to GetURL()
* Implement use of `--entrypoint=<cmd>` to force execution of command: closes 306
* Implement use of `--entrypoint=""` to bypass entrypoint in metadata: closes 306

* Method Unshare.unshare os.strerror() takes one argument: closes 254
* Add unit test for 254
* Method chown udocker.utils.fileutil FileUtil: closes 276
* Several fixes of unit tests and pylint
* Fix confusion between exit code 0 and inferred False
* Dereference on `safe_prefixes`
* untar exclude dev
* Fix rmi for referenced layers
* Set default for `PROOT_TMP_DIR`
* sysdir mountpoint not found and set tmpdir
* Update installation instructions
* Improve `oskernel_isgreater()`
* Improve `osinfo()`
* Fix repository login/logout
* Improve keystore logic
* Fix pull /v2