Normalize the repository by running `opam-admin check -n`

This makes the repository format compatible with OPAM 1.1 only.
Add external dependencies for Ubuntu and Debian.

These depexts can be queried via `opam install -e ubuntu <pkg>` to
let external helper scripts satisfy OS dependencies before starting
the installation.
Aio: add ubuntu depext tag
Alsa: add depexts for Ubuntu/Debian (#1304)
Ao: add depexts for Ubuntu/Debian (#1304)
ssl: add ubuntu/debian extdep (#1304)
bitstring: add time depext for ubuntu/debian (#1304)
Ao: fix depext name
Cryptokit: add Ubuntu/Debian depexts (#1304)
lablgtk: add Ubuntu/Debian depexts (#1304)
Mysql: add depexts (#1304)
ocaml-usb: add depexts for ubuntu/debian (#1304)
krb5: add depexts for Ubuntu/Debian (#1304)
cairo: missing autoconf depext (#1304)
conf-libpng: add deppexts (#1304)
ftgl: add ubuntu/debian depexts (#1304)
libffi: add depext on ubuntu/debian (#1304)
bjack: add ubuntu/debian depexts (#1304)
conf-libcurl: add depext for ubuntu/debian (#1304)
conf-gsl: add ubuntu/debian depexts (#1304)