Add pseudo --server command

Ref https://github.com/sbt/sbt/issues/5665

This adds `--server` command that is immediately filtered out in Main.scala.
The purpose of `--server` is so we can invoke thin client from `sbt` script at some point in the future when Bash script can parse `project/build.properties`.

`sbtn` would need to call `sbt` again to start the server, and at that point the shell script would need to actually invoke the server. The intent of `--server` is to be used as the tie breaker.

Also build users may want to sometimes call `sbt --server`.

main-command/src/main/scala/sbt/BasicCommandStrings.scala

@@ -200,6 +200,10 @@ $AliasCommand name=
 	Starts the server if it has not been started. This is intended to be used with
 	-Dsbt.server.autostart=false."""
 
+  def ServerDetailed: String =
+    "--server always runs sbt in not-daemon mode."
+  def DashDashServer: String = "--server"
+
   def OldShell: String = "oldshell"
   def OldShellDetailed = "Provides an interactive prompt from which commands can be run."
 

main-command/src/main/scala/sbt/internal/client/NetworkClient.scala

@@ -298,7 +298,8 @@ class NetworkClient(
             term.isSupershellEnabled
           ).mkString(",")
 
-        val cmd = arguments.sbtScript +: arguments.sbtArguments :+ BasicCommandStrings.CloseIOStreams
+        val cmd = List(arguments.sbtScript) ++ arguments.sbtArguments ++
+          List(BasicCommandStrings.CloseIOStreams, BasicCommandStrings.DashDashServer)
         val processBuilder =
           new ProcessBuilder(cmd: _*)
             .directory(arguments.baseDirectory)

main/src/main/scala/sbt/Main.scala

@@ -59,7 +59,7 @@ private[sbt] object xMain {
   }
   private[sbt] def run(configuration: xsbti.AppConfiguration): xsbti.MainResult = {
     try {
-      import BasicCommandStrings.{ DashClient, DashDashClient, runEarly }
+      import BasicCommandStrings.{ DashClient, DashDashClient, DashDashServer, runEarly }
       import BasicCommands.early
       import BuiltinCommands.defaults
       import sbt.internal.CommandStrings.{ BootCommand, DefaultsCommand, InitCommand }
@@ -71,7 +71,9 @@ private[sbt] object xMain {
       }
       // if we detect -Dsbt.client=true or -client, run thin client.
       val clientModByEnv = SysProp.client
-      val userCommands = configuration.arguments.map(_.trim)
+      val userCommands = configuration.arguments
+        .map(_.trim)
+        .filterNot(_ == DashDashServer)
       val isClient: String => Boolean = cmd => (cmd == DashClient) || (cmd == DashDashClient)
       val isBsp: String => Boolean = cmd => (cmd == "-bsp") || (cmd == "--bsp")
       if (userCommands.exists(isBsp)) {