allow for deferred init of `UnlinkGuard`
TEST: T_UnlinkGuard::DeferredInit
Refactor: `CommandCreateTag` for code reuse
add `cvmfs_swissknife tag_remove`
add assertion for upload
add `History::Tag::GetChannelName`
add implementation for `cvmfs_swissknife tag_list`
add `cvmfs_swissknife tag_info`
read only access based on signed manifest