Update versioned build-dependency on debhelper accordingly.

Thanks DUCK for the hint!

Update debian/copyright format URL to HTTPS.

Set myself personally there instead.

Closes: #899691

Issue found by DUCK.

No additional changes were required.

It seems that at least Iceweasel can't connect to that FTP server, but
other FTP clients (e.g. ncftp, uscan) still can.

Remove quilt build-dependency and quilt usage from debian/rules

Drop manually build $CFLAGS.

Update versioned debhelper build-dependency