spark-daemon.sh PID reuse check fails on long classpath

sbin/spark-daemon.sh uses ps -p "$TARGET_PID" -o args= to figure out whether the process running with the expected PID is actually a Spark daemon. When running with a large classpath, the output of ps gets truncated and the check fails spuriously.

I think we should weaken the check to see if it's a java command (which is something we do in other parts of the script) rather than looking for the specific main class name. This means that SPARK-4832 might happen under a slightly broader range of circumstances (a java program happened to reuse the same PID), but it seems worthwhile compared to failing consistently with a large classpath.