Based on work from Lorenzo, updated by Iago, Alban, Alessandro and
Michael.

This PR adds connection tracking using eBPF. This feature is not enabled by default.
For now, you can enable it by launching scope with the following command:

```
sudo ./scope launch --probe.ebpf.connections=true
```

This patch allows scope to get notified of every connection event,
without relying on the parsing of /proc/$pid/net/tcp{,6} and
/proc/$pid/fd/*, and therefore improve performance.

We vendor https://github.com/iovisor/gobpf in Scope to load the
pre-compiled ebpf program and https://github.com/weaveworks/tcptracer-bpf
to guess the offsets of the structures we need in the kernel. In this
way we don't need a different pre-compiled ebpf object file per kernel.
The pre-compiled ebpf program is included in the vendoring of
tcptracer-bpf.

The ebpf program uses kprobes/kretprobes on the following kernel functions:
- tcp_v4_connect
- tcp_v6_connect
- tcp_set_state
- inet_csk_accept
- tcp_close

It generates "connect", "accept" and "close" events containing the
connection tuple but also pid and netns.
Note: the IPv6 events are not supported in Scope and thus not passed on.

probe/endpoint/ebpf.go maintains the list of connections. Similarly to
conntrack, it also keeps the dead connections for one iteration in order
to report short-lived connections.

The code for parsing /proc/$pid/net/tcp{,6} and /proc/$pid/fd/* is still
there and still used at start-up because eBPF only brings us the events
and not the initial state. However, the /proc parsing for the initial
state is now done in foreground instead of background, via
newForegroundReader().

NAT resolution on connections from eBPF works in the same way as it did
on connections from /proc: by using conntrack. One of the two conntrack
instances is only started to get the initial state and then it is
stopped since eBPF detects short-lived connections.

The Scope Docker image size comparison:
- weaveworks/scope in current master:  22 MB (compressed),  68 MB
  (uncompressed)
- weaveworks/scope with this patchset: 23 MB (compressed), 69 MB
  (uncompressed)

Fixes #1168 (walking /proc to obtain connections is very expensive)

Fixes #1260 (Short-lived connections not tracked for containers in
shared networking namespaces)

Fixes #1962 (Port ebpf tracker to Go)

Fixes #1961 (Remove runtime kernel header dependency from ebpf tracker)