+act #15956 Allow _ in ActorSystem name

(cherry picked from commit 7ef1334192de6f2c0cffbb81be620cdd56110ad5)

akka-actor-tests/src/test/scala/akka/actor/ActorPathSpec.scala

@@ -17,7 +17,7 @@ class ActorPathSpec extends WordSpec with Matchers {
     }
 
     "support parsing remote paths" in {
-      val remote = "akka://sys@host:1234/some/ref"
+      val remote = "akka://my_sys@host:1234/some/ref"
       ActorPath.fromString(remote).toString should be(remote)
     }
 

akka-actor-tests/src/test/scala/akka/actor/ActorSystemSpec.scala

@@ -144,8 +144,8 @@ class ActorSystemSpec extends AkkaSpec(ActorSystemSpec.config) with ImplicitSend
     "reject invalid names" in {
       for (
         n ← Seq(
-          "hallo_welt",
           "-hallowelt",
+          "_hallowelt",
           "hallo*welt",
           "hallo@welt",
           "hallo#welt",
@@ -158,7 +158,7 @@ class ActorSystemSpec extends AkkaSpec(ActorSystemSpec.config) with ImplicitSend
     }
 
     "allow valid names" in {
-      shutdown(ActorSystem("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-"))
+      shutdown(ActorSystem("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_"))
     }
 
     "support extensions" in {

akka-actor/src/main/scala/akka/actor/ActorSystem.scala

@@ -521,7 +521,7 @@ abstract class ExtendedActorSystem extends ActorSystem {
 
 private[akka] class ActorSystemImpl(val name: String, applicationConfig: Config, classLoader: ClassLoader, defaultExecutionContext: Option[ExecutionContext]) extends ExtendedActorSystem {
 
-  if (!name.matches("""^[a-zA-Z0-9][a-zA-Z0-9-]*$"""))
+  if (!name.matches("""^[a-zA-Z0-9][a-zA-Z0-9-_]*$"""))
     throw new IllegalArgumentException(
       "invalid ActorSystem name [" + name +
         "], must contain only word characters (i.e. [a-zA-Z0-9] plus non-leading '-')")