iptables package has a function `detectIptables()` called to initialize
some local variables. Since v20.10.0, it first looks for iptables bin,
then ip6tables and finally it checks what iptables flags are available
(including -C). It early exits when ip6tables isn't available, and
doesn't execute the last check.

To remove port mappings (eg. when a container stops/dies), Docker
first checks if those NAT rules exist and then deletes them. However, in
the particular case where there's no ip6tables bin available, iptables
`-C` flag is considered unavailable and thus it looks for NAT rules by
using some substring matching. This substring matching then fails
because `iptables -t nat -S POSTROUTING` dumps rules in a slighly format
than what's expected.

For instance, here's what `iptables -t nat -S POSTROUTING` dumps:

```
-A POSTROUTING -s 172.18.0.2/32 -d 172.18.0.2/32 -p tcp -m tcp --dport 9999 -j MASQUERADE
```

And here's what Docker looks for:

```
POSTROUTING -p tcp -s 172.18.0.2 -d 172.18.0.2 --dport 9999 -j MASQUERADE
```

Because of that, those rules are considered non-existant by Docker and
thus never deleted. To fix that, this change reorders the code in
`detectIptables()`.

Fixes moby#42127.

Signed-off-by: Albin Kerouanton <[email protected]>
(cherry picked from commit af7236f)
Signed-off-by: Sebastiaan van Stijn <[email protected]>

The former was doing some checks and logging warnings, whereas
the latter was doing the same checks but to set some internal variables.
As both are called only once and from the same place, there're now
merged together.

Signed-off-by: Albin Kerouanton <[email protected]>
(cherry picked from commit 205e527)
Signed-off-by: Sebastiaan van Stijn <[email protected]>

iptables -C flag was introduced in v1.4.11, which was released ten
years ago. Thus, there're no more Linux distributions supported by
Docker using this version. As such, this commit removes the old way of
checking if an iptables rule exists (by using substring matching).

Signed-off-by: Albin Kerouanton <[email protected]>
(cherry picked from commit 799cc14)
Signed-off-by: Sebastiaan van Stijn <[email protected]>