libpod: Add pasta networking mode

Conceptually equivalent to networking by means of slirp4netns(1),
with a few practical differences:

- pasta(1) forks to background once networking is configured in the
  namespace and quits on its own once the namespace is deleted:
  file descriptor synchronisation and PID tracking are not needed

- port forwarding is configured via command line options at start-up,
  instead of an API socket: this is taken care of right away as we're
  about to start pasta

- there's no need for further selection of port forwarding modes:
  pasta behaves similarly to containers-rootlessport for local binds
  (splice() instead of read()/write() pairs, without L2-L4
  translation), and keeps the original source address for non-local
  connections like slirp4netns does

- IPv6 is not an experimental feature, and enabled by default. IPv6
  port forwarding is supported

- by default, addresses and routes are copied from the host, that is,
  container users will see the same IP address and routes as if they
  were in the init namespace context. The interface name is also
  sourced from the host upstream interface with the first default
  route in the routing table. This is also configurable as documented

- sandboxing and seccomp(2) policies cannot be disabled

- only rootless mode is supported.

See https://passt.top for more details about pasta.

Also add a link to the maintained build of pasta(1) manual as valid
in the man page cross-reference checks: that's where the man page
for the latest build actually is -- it's not on Github and it doesn't
match any existing pattern, so add it explicitly.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

libpod/networking_linux.go

@@ -563,6 +563,9 @@ func (r *Runtime) configureNetNS(ctr *Container, ctrNS ns.NetNS) (status map[str
 	if ctr.config.NetMode.IsSlirp4netns() {
 		return nil, r.setupSlirp4netns(ctr, ctrNS)
 	}
+	if ctr.config.NetMode.IsPasta() {
+		return nil, r.setupPasta(ctr, ctrNS)
+	}
 	networks, err := ctr.networks()
 	if err != nil {
 		return nil, err