tests: repo handle: test the useragent string.

Also see 36f1d76, except the property only became available for reading in
librepo today.
fix configuration of per-repo excludes.

Also respect CLI's '--disableexcludes'.

Related: RhBug:947258
doc: fix failing documentation build after dnf.yum.metalink is gone.
doc: the excludes behavior (RhBug:947258)