This abstracts the bit flag checking for repository
references.

Currently if a discovery event would define a nickname it is not passed
by the add repository method resulting in an updatesite to have an empty
name.

This adds a new method that allows to pass a custom RepositoryInfo so
the nickname can be set.

…stead of user

Currently there is a quite surprising behavior, that if a user adds an
updatesite that contains a referenced repository, that after an
update-check additional repositories are visible. Even worse these are
now used additionally to check for updates and if they contain other
references these are also added and so on. This can result not only in a
long list of sites the user never has added and has no clue where they
are coming from but even pulling in unwanted or conflicting updates.

This now distinguishes two cases:

1) A repository is discovered by the RepositoryAction it is handled as a
user added repository and becomes visible
2) A repository is discovered by a reference in that case it is handled
as a system repository and not becomes visible

that way the list of user visible repositories stay clean from
referenced repositories and unexpected side effects on update checks.