[SPARK-6952] Handle long args when detecting PID reuse

[punya]

sbin/spark-daemon.sh used

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.

This weakens 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.

[srowen]

LGTM for the reasoning given here.

[SparkQA]

Test build #30383 has finished for PR 5535 at commit 7ea12d1.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • case class UnresolvedAttribute(nameParts: Seq[String])
  • trait CaseConversionExpression
  • final class UTF8String extends Ordered[UTF8String] with Serializable
  • case class Exchange(
  • case class SortMergeJoin(
• This patch does not change any dependencies.

[punya]

Fairly sure I didn't add any of those classes in 2 lines of shell script 😛