[SPARK-4057] Use -agentlib instead of -Xdebug in sbt-launch-lib.bash for debugging

In -launch-lib.bash, -Xdebug option is used for debugging. We should use -agentlib option for Java 6+.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2904 from sarutak/SPARK-4057 and squashes the following commits:

39b5320 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into SPARK-4057
26b4af8 [Kousuke Saruta] Improved java option for debugging

sbt/sbt-launch-lib.bash

@@ -104,7 +104,7 @@ addResidual () {
   residual_args=( "${residual_args[@]}" "$1" )
 }
 addDebugger () {
-  addJava "-Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=$1"
+  addJava "-agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=$1"
 }
 
 # a ham-fisted attempt to move some memory settings in concert