config: extract prepend_installroot() to a public method.
progress: reformat and reword docstrings
progress: add "err" argument to end() callback, merge with failure()

- merged two code paths
- use the new librepo API, where the failurecb arg was dropped, too.
- updated unit tests, it now checks the failure mode too.
always enable LRO_FASTESTMIRROR
tests: skip RepoTest.test_const_install.

activate_sack() fails under the Koji environment as /run/user is not writable
for the metadata lock.
New version: 0.4.2
build: 0.4.2-1
refactor: Repo._handle_new_remote()

removed: Repo._handle_new_pkg_download()
removed: Repo._mirror_setup_args()
removed: Repo._no_mirror_setup_args()
removed: _Handle.new_remote()

Also fixes a subtle bug that _handle_new_pkg_download()
used only the first baseurl.