implement creation of SWARM-NAT chain

this is done by two functions:
Test-IptablesChain
Add-IptablesChain

expose_forwards.ps1

@@ -1,6 +1,39 @@
 #Requires -Modules powershell-yaml
 $ErrorActionPreference = 'Stop'
 
+function Test-IptablesChain {
+    param (
+        [string]$Chain,
+        [string]$Table = 'nat'
+    )
+
+    $output = iptables -t $Table -S $Chain
+    $reference = '-N {0}' -f $Chain
+
+    #check $output in case $Chain has no rules
+    #otherwise check the first line with $output[0]
+    $output.Count -gt 0 `
+    -and ($output -eq $reference `
+          -or `
+          $output[0] -eq $reference)
+}
+function Add-IptablesChain {
+    param (
+        [string]$Chain,
+        [string]$Table = 'nat'
+    )
+
+    if(-not (Test-IptablesChain -Chain $Chain)) {
+        iptables -t $Table -N $Chain
+    }
+}
+
+# setup SWARM-NAT chain
+$chain = 'SWARM-NAT'
+Write-Output ('Create chain {0}' -f $chain)
+Add-IptablesChain -Chain $chain
+
+
 foreach($yaml in (Get-ChildItem -Filter '*.yml')) {
     Write-Output ('Processing {0}' -f $yaml)
     $definition = Get-Content -Path $yaml -Raw | ConvertFrom-Yaml