Allow selecting mirror 1st time when adding packman repo

[asdil12]

This will look something like this:

You have selected package: Packman Essentials ? | 3.5 | x86_64
Adding packman repo
1. ftp.fau.de                 - University of Erlangen, Germany  -  1h sync
2. ftp.halifax.rwth-aachen.de - University of Aachen, Germany    -  1h sync
3. ftp.gwdg.de                - University of Göttingen, Germany -  4h sync
4. mirror.karneval.cz         - TES Media, Czech Republic        -  1h sync
5. mirrors.aliyun.com         - Alibaba Cloud, China             - 24h sync
Pick a mirror near your location (0 to quit): 2
Import package signing key 'PackMan Project (signing key) <packman@links2linux.de>' (Y/n) y