my-env: Preserve http_proxy and ftp_proxy variables

There are few build scripts which set them to `nodtd.invalid` to disable
downloading files by buildscript. But for user environment we should
restore original values
Revert "Revert "Merge pull request #3182 from wkennington/master.ipv6""

This reverts commit ea8910652fcecbcd4f21aa1c66b1a0a239408b04.
Revert "Revert "Fix syntax error in nixos/lib/build-vms.nix, introduced by 86c0f8c""

This reverts commit 2f697bf6931b24cdd428e22effbf6427a85afd42.
nixos/network-interfaces: Fix bug in converting old ipAddresses
Merge pull request #3908 from wkennington/master.ip

Reapply the multi-ip code
Merge pull request #3257 from tailhook/myenv-proxy

my-env: Preserve http_proxy and ftp_proxy variables
Add IDs for uhub service
uhub: initial package for version 0.4.1
uhub: initial service expression