SI-6502 Repl reset/replay take settings args

The reset and replay commands take arbitrary command line args.
When settings args are supplied, the compiler is recreated.

For uniformity, the settings command performs only the usual
arg parsing: use -flag:true instead of +flag, and clearing a
setting is promoted to the command line, so that -Xlint: is not
an error but clears the flags.

```
scala> maqicode.Test main null
<console>:8: error: not found: value maqicode
              maqicode.Test main null
              ^

scala> :reset -classpath/a target/scala-2.11/sample_2.11-1.0.jar
Resetting interpreter state.
Forgetting all expression results and named terms: $intp

scala> maqicode.Test main null
Hello, world.

scala> val i = 42
i: Int = 42

scala> s"$i is the loneliest numbah."
res1: String = 42 is the loneliest numbah.

scala> :replay -classpath ""
Replaying: maqicode.Test main null
Hello, world.

Replaying: val i = 42
i: Int = 42

Replaying: s"$i is the loneliest numbah."
res1: String = 42 is the loneliest numbah.

scala> :replay -classpath/a ""
Replaying: maqicode.Test main null
<console>:8: error: not found: value maqicode
              maqicode.Test main null
              ^

Replaying: val i = 42
i: Int = 42

Replaying: s"$i is the loneliest numbah."
res1: String = 42 is the loneliest numbah.
```

Clearing a clearable setting:
```
scala> :reset -Xlint:missing-interpolator
Resetting interpreter state.

scala> { val i = 42 ; "$i is the loneliest numbah." }
<console>:8: warning: possible missing interpolator: detected interpolated identifier `$i`
              { val i = 42 ; "$i is the loneliest numbah." }
                             ^
res0: String = $i is the loneliest numbah.

scala> :reset -Xlint:
Resetting interpreter state.
Forgetting this session history:

{ val i = 42 ; "$i is the loneliest numbah." }

scala> { val i = 42 ; "$i is the loneliest numbah." }
res0: String = $i is the loneliest numbah.

```

2 files changed, 24 insertions, 7 deletions

diff --git a/src/compiler/scala/tools/nsc/settings/AbsSettings.scala b/src/compiler/scala/tools/nsc/settings/AbsSettings.scala
index 4727e6d867..060a24d8d4 100644
--- a/src/compiler/scala/tools/nsc/settings/AbsSettings.scala
+++ b/src/compiler/scala/tools/nsc/settings/AbsSettings.scala
@@ -35,7 +35,11 @@ trait AbsSettings extends scala.reflect.internal.settings.AbsSettings {
     case s: AbsSettings => this.userSetSettings == s.userSetSettings
     case _              => false
   }
-  override def toString() = "Settings {\n%s}\n" format (userSetSettings map ("  " + _ + "\n")).mkString
+  override def toString() = {
+    val uss    = userSetSettings
+    val indent = if (uss.nonEmpty) " " * 2 else ""
+    uss.mkString(f"Settings {%n$indent", f"%n$indent", f"%n}%n")
+  }
   def toConciseString = userSetSettings.mkString("(", " ", ")")
 
   def checkDependencies =
diff --git a/src/compiler/scala/tools/nsc/settings/MutableSettings.scala b/src/compiler/scala/tools/nsc/settings/MutableSettings.scala
index bbe21477cb..23611bb629 100644
--- a/src/compiler/scala/tools/nsc/settings/MutableSettings.scala
+++ b/src/compiler/scala/tools/nsc/settings/MutableSettings.scala
@@ -125,14 +125,26 @@ class MutableSettings(val errorFn: String => Unit)
         case Some(cmd)  => setter(cmd)(args)
       }
 
-    // if arg is of form -Xfoo:bar,baz,quux
-    def parseColonArg(s: String): Option[List[String]] = {
-      val (p, args) = StringOps.splitWhere(s, _ == ':', doDropIndex = true) getOrElse (return None)
-
-      // any non-Nil return value means failure and we return s unmodified
-      tryToSetIfExists(p, (args split ",").toList, (s: Setting) => s.tryToSetColon _)
+    // -Xfoo: clears Clearables
+    def clearIfExists(cmd: String): Option[List[String]] = lookupSetting(cmd) match {
+      case Some(c: Clearable) => c.clear() ; Some(Nil)
+      case Some(s)            => s.errorAndValue(s"Missing argument to $cmd", None)
+      case None               => None
     }
 
+    // if arg is of form -Xfoo:bar,baz,quux
+    // the entire arg is consumed, so return None for failure
+    // any non-Nil return value means failure and we return s unmodified
+    def parseColonArg(s: String): Option[List[String]] =
+      if (s endsWith ":") {
+        clearIfExists(s.init)
+      } else {
+        for {
+          (p, args) <- StringOps.splitWhere(s, _ == ':', doDropIndex = true)
+          rest      <- tryToSetIfExists(p, (args split ",").toList, (s: Setting) => s.tryToSetColon _)
+        } yield rest
+      }
+
     // if arg is of form -Xfoo or -Xfoo bar (name = "-Xfoo")
     def parseNormalArg(p: String, args: List[String]): Option[List[String]] =
       tryToSetIfExists(p, args, (s: Setting) => s.tryToSet _)
@@ -532,6 +544,7 @@ class MutableSettings(val errorFn: String => Unit)
     def prepend(s: String) = prependPath.value = join(s, prependPath.value)
     def append(s: String) = appendPath.value = join(appendPath.value, s)
 
+    override def isDefault = super.isDefault && prependPath.isDefault && appendPath.isDefault
     override def value = join(
       prependPath.value,
       super.value,