Non-root owned symlink causes install failure

[ffesti]

This is a continuation of #3100. Unsafe symlinks are detected during installation and create a failure. This is the right thing to do in case we encounter such things at this phase. But we really need to check for this in advance during the transaction check and not even start the transaction if such symlinks are found. The failure during the transaction is only acceptable if someone tries to escalate their privileges exploiting a race condition.