Better document and check requirements for --apt_proxy

See http://bugs.debian.org/623443 for the corresponding apt issue.

Also add a dependency on libdata-validate-uri-perl which is used to do
the syntax check.
Use Data::Validate::{Domain,IP} for IP addresses and hostname checks

Add corresponding dependencies on libdata-validate-{domain,ip}-perl.
Mention that #715340 is fixed by 1de424a6
Bump debhelper compatibility to 9

Update versioned debhelper build-dependency
Add dependency on ${perl:Depends} to fix dpkg-gencontrol warning
TODO: Generate ECDSA host keys where possible