Implement 'suppressServer' setting, for builds and plugins that prefer to be conservative about exposure to other processes.

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

@@ -39,6 +39,12 @@ object BasicKeys {
                                  "The wire protocol for the server command.",
                                  10000)
 
+  val suppressServer =
+    AttributeKey[Boolean](
+      "suppressServer",
+      "Running the server will be suppressed if 'suppressServer is explicitly set to true.",
+      10000)
+
   // Unlike other BasicKeys, this is not used directly as a setting key,
   // and severLog / logLevel is used instead.
   private[sbt] val serverLogLevel =