Enable specifying sshd subsystem to use via -Subsystem (#6603)

The current implementation of PSRP over SSH only enables remoting to one version of PowerShell Core 6.
sshd_config allows specifying multiple subsystems that can start different versions of PowerShell Core 6 (or same versions with different parameters).
Enable use of a new -Subsystem parameter to specify the subsystem to use with ssh.

src/System.Management.Automation/engine/remoting/common/RunspaceConnectionInfo.cs

@@ -1867,6 +1867,15 @@ namespace System.Management.Automation.Runspaces
             set;
         }
 
+        /// <summary>
+        /// Subsystem to use
+        /// </summary>
+        private string Subsystem
+        {
+            get;
+            set;
+        }
+
         #endregion
 
         #region Constructors
@@ -1894,6 +1903,7 @@ namespace System.Management.Automation.Runspaces
             this.ComputerName = computerName;
             this.KeyFilePath = keyFilePath;
             this.Port = DefaultPort;
+            this.Subsystem = DefaultSubsystem;
         }
 
         /// <summary>
@@ -1903,15 +1913,18 @@ namespace System.Management.Automation.Runspaces
         /// <param name="computerName">Computer Name</param>
         /// <param name="keyFilePath">Key File Path</param>
         /// <param name="port">Port number for connection (default 22)</param>
+        /// <param name="subsystem">Subsystem to use (default 'powershell')</param>
         public SSHConnectionInfo(
             string userName,
             string computerName,
             string keyFilePath,
-            int port) : this(userName, computerName, keyFilePath)
+            int port,
+            string subsystem) : this(userName, computerName, keyFilePath)
         {
             ValidatePortInRange(port);
 
             this.Port = (port != 0) ? port : DefaultPort;
+            this.Subsystem = (String.IsNullOrEmpty(subsystem)) ? DefaultSubsystem : subsystem;
         }
 
         #endregion
@@ -1965,6 +1978,7 @@ namespace System.Management.Automation.Runspaces
             newCopy.UserName = this.UserName;
             newCopy.KeyFilePath = this.KeyFilePath;
             newCopy.Port = this.Port;
+            newCopy.Subsystem = this.Subsystem;
 
             return newCopy;
         }
@@ -2040,14 +2054,14 @@ namespace System.Management.Automation.Runspaces
                 }
 
                 arguments = (string.IsNullOrEmpty(domainName)) ?
-                    string.Format(CultureInfo.InvariantCulture, @"-i ""{0}"" {1}@{2} -p {3} -s powershell", this.KeyFilePath, userName, this.ComputerName, this.Port) :
-                    string.Format(CultureInfo.InvariantCulture, @"-i ""{0}"" -l {1}@{2} {3} -p {4} -s powershell", this.KeyFilePath, userName, domainName, this.ComputerName, this.Port);
+                    string.Format(CultureInfo.InvariantCulture, @"-i ""{0}"" {1}@{2} -p {3} -s {4}", this.KeyFilePath, userName, this.ComputerName, this.Port, this.Subsystem) :
+                    string.Format(CultureInfo.InvariantCulture, @"-i ""{0}"" -l {1}@{2} {3} -p {4} -s {5}", this.KeyFilePath, userName, domainName, this.ComputerName, this.Port, this.Subsystem);
             }
             else
             {
                 arguments = (string.IsNullOrEmpty(domainName)) ?
-                    string.Format(CultureInfo.InvariantCulture, @"{0}@{1} -p {2} -s powershell", userName, this.ComputerName, this.Port) :
-                    string.Format(CultureInfo.InvariantCulture, @"-l {0}@{1} {2} -p {3} -s powershell", userName, domainName, this.ComputerName, this.Port);
+                    string.Format(CultureInfo.InvariantCulture, @"{0}@{1} -p {2} -s {3}", userName, this.ComputerName, this.Port, this.Subsystem) :
+                    string.Format(CultureInfo.InvariantCulture, @"-l {0}@{1} {2} -p {3} -s {4}", userName, domainName, this.ComputerName, this.Port, this.Subsystem);
             }
 
             System.Diagnostics.ProcessStartInfo startInfo = new System.Diagnostics.ProcessStartInfo(
@@ -2082,6 +2096,11 @@ namespace System.Management.Automation.Runspaces
         /// </summary>
         private const int DefaultPort = 22;
 
+        /// <summary>
+        /// Default value for subsystem
+        /// </summary>
+        private const string DefaultSubsystem = "powershell";
+
         #endregion
 
         #region SSH Process Creation