[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

sbin/spark-daemon.sh

@@ -129,7 +129,7 @@ run_command() {
 
   if [ -f "$pid" ]; then
     TARGET_ID="$(cat "$pid")"
-    if [[ $(ps -p "$TARGET_ID" -o args=) =~ $command ]]; then
+    if [[ $(ps -p "$TARGET_ID" -o comm=) =~ "java" ]]; then
       echo "$command running as process $TARGET_ID.  Stop it first."
       exit 1
     fi
@@ -163,7 +163,7 @@ run_command() {
   echo "$newpid" > "$pid"
   sleep 2
   # Check if the process has died; in that case we'll tail the log so the user can see
-  if [[ ! $(ps -p "$newpid" -o args=) =~ $command ]]; then
+  if [[ ! $(ps -p "$newpid" -o comm=) =~ "java" ]]; then
     echo "failed to launch $command:"
     tail -2 "$log" | sed 's/^/  /'
     echo "full log in $log"