debian_devscripts: 2.15.4 -> 2.15.8

Also:
- add some previously missing dependencies and patch /usr paths
- fix fetching of https:// URIs with perlPackages.LWPProtocolHttps
- remove the bash completion work-around (no longer needed)
- add newly required make flags
- change licence from string to attribute (still unspecified)

CC maintainer @raskin.