Use iptables-save alongside explicit lists of tables

This was inspired by the recent addition of the mangle table in
iptables output for subctl gather. In addition to the current logging
of manually-specified tables (in a format which is easy to read), it
uses iptables-save to automatically determine all the active tables.
This ensures that if a user's setup relies on other tables, developers
can still determine what's happening without having to go back to the
user to run additional iptables commands.

Signed-off-by: Stephen Kitt <[email protected]>

internal/gather/cni.go

@@ -52,6 +52,7 @@ var ipTablesCmds = map[string]string{
 	"iptables":        "iptables -L -n -v --line-numbers",
 	"iptables-nat":    "iptables -L -n -v --line-numbers -t nat",
 	"iptables-mangle": "iptables -L -n -v --line-numbers -t mangle",
+	"iptables-save":   "iptables-save -c",
 }
 
 var libreswanCmds = map[string]string{