Commit 1: API: support GET /info

e.g.

$ rootlessctl info -json
{
    "apiVersion": "1.1.0",
    "version": "0.13.2+dev",
    "stateDir": "/tmp/rootlesskit751356081",
    "childPID": 163654,
    "networkDriver": {
        "driver": "slirp4netns",
        "dns": [
            "10.0.2.3"
        ]
    },
    "portDriver": {
        "driver": "builtin",
        "protos": [
            "tcp",
            "udp"
        ]
    }
}

Inspecting .portDriver.protos is required for implementing #231 (comment)

Commit 2: Port API: support specifying IP version explicitly ("tcp4", "tcp6")

Fix #231

See ./docs.port.md

Specifying `0.0.0.0:8080:80/tcp` may cause listening on IPv6 as well as on IPv4.
Same applies to `[::]:8080:80/tcp`.

This behavior may sound weird but corresponds to [Go's behavior](https://github.com/golang/go/commit/071908f3d809245eda
42bf6eab071c323c67b7d2),
so this is not a bug.

To specify IPv4 explicitly, use `tcp4` instead of `tcp`, e.g., `0.0.0.0:8080:80/tcp4`.
To specify IPv6 explicitly, use `tcp6`, e.g., `[::]:8080:80/tcp6`.

The `tcp4` and `tcp6` forms were introduced in RootlessKit v0.14.0.
The `tcp6` is currently supported only for `builtin` port driver.

Commit 3: rootlesskit-docker-proxy: support libnetwork >= 20201216 convention

The -proto argument of docker-proxy is like "tcp", but we need to convert it to "tcp4" or "tcp6" explicitly when calling RootlessKit API, for libnetwork >= 20201216.

If the port driver does not support "tcp6" (especially when the port driver is slirp4netns), rootlesskit-docker-proxy skips exposing the port via RootlessKit API, without showing an error.

(We can't raise an error here, because docker run -p 8080:80 always causes rootlesskit-docker-proxy -host-ip :: as well as r-d-p -h-i 0.0.0.0)

See https://github.com/moby/libnetwork/pull/2604/files#diff-8fa48beed55dd033bf8e4f8c40b31cf69d0b2cc5d4bb53cde8594670ea6c938aR20

See also #231

Using this version of rootlesskit-docker-proxy with libnetwork < 20201216 is also fine, because Rootless Docker had never officially supported IPv6.