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

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.

Author: Punya Biswal <pbiswal@palantir.com>

Closes #5535 from punya/feature/SPARK-6952 and squashes the following commits:

7ea12d1 [Punya Biswal] Handle long args when detecting PID reuse