ZOOKEEPER-4636: Fix zkServer.sh for AIX

AIX "echo" does not support "-n" argument.

Author: henk.wiedig@t-systems.com <henk.wiedig@t-systems.com>

Reviewers: Enrico Olivelli <eolivelli@apache.org>, maoling <maoling@apache.org>

Closes #1945 from henkwiedig/fix-aix-zkserver

bin/zkServer.sh

@@ -171,6 +171,9 @@ start)
       *solaris*)
         /bin/echo "${!}\\c" > "$ZOOPIDFILE"
         ;;
+      aix*)
+        /bin/echo "$!\c" > "$ZOOPIDFILE"
+        ;;
       *)
         /bin/echo -n $! > "$ZOOPIDFILE"
         ;;