Fix usage printing for compiler plugins

compiler/plugin-api/src/org/jetbrains/kotlin/compiler/plugin/CliOptions.kt

@@ -45,8 +45,10 @@ public fun cliPluginUsageString(pluginId: String, options: Collection<CliOption>
             "\n" + " ".repeat(MAX_OPTION_WIDTH + LEFT_INDENT + 1)
         } else " ".repeat(1 + MAX_OPTION_WIDTH - name.length())
 
-        val modifiers = (if (it.required) "required" else "") + (if (it.allowMultipleOccurrences) "multiple" else "")
-        val modifiersEnclosed = if (modifiers.isNotEmpty()) " ($modifiers)" else ""
+        val modifiers = listOf(
+                if (it.required) "required" else null,
+                if (it.allowMultipleOccurrences) "multiple" else null).filterNotNull()
+        val modifiersEnclosed = if (modifiers.isEmpty()) "" else " (${modifiers.joinToString()})"
 
         " ".repeat(LEFT_INDENT) + name + margin + it.description + modifiersEnclosed
     }

compiler/testData/cli/jvm/pluginSimpleUsage.out

@@ -1,7 +1,7 @@
 ERROR: Required plugin option not present: org.jetbrains.kotlin.android:androidRes
 
 Plugin "org.jetbrains.kotlin.android" usage:
-  androidRes <path>          Android resources path (required)
+  androidRes <path>          Android resources path (required, multiple)
   androidManifest <path>     Android manifest file (required)
   supportV4 <path>           Support android-v4 library