IP configuration --v4 ip/mask instead of --ip X --netmask Y

[hannesm]

instead of using V4[V6].ip & netmask (read: two command-line arguments), use a single one: ip/netmask). This leads to fewer potential error conditions (what happens if a is provided, but not b), and is pretty standard.

The gateway can be a separate key since it is optional anyways (it is perfectly good to have a unikernel without a default gateway -- e.g. a local syslog collector, a DHCP server, ...).

Work only to be done once #643 is in to avoid conflicts and needless rebasing.

[hannesm]

(but default gateway that isn't on the network should be a runtime error, probably)

should error as early as we can!

[yomimono]

Is this actually the right thing to do for ipv6? See the last couple comments in #557

[yomimono]

The current behavior came in with #643 , which addressed in part #557 .

[mato]

The current behaviour is actually two arguments, --ip=IP and --network=NETWORK/MASK.

This syntax violates POLA and is unnecessarily hard to implement in things like docker-unikernel-runner, so I'd like to bump this issue. Can we change to just --ip=IP/MASK please? :-)