Startup: Add ES_HOME to ES_INCLUDE search path

With this change, the elasticsearch script can be linked to another path
without having to set ES_INCLUDE to match the installation path.
Previously, the elasticsearch would find ES_HOME correctly even if linked
but could not find the include script, and finding it would be expected
behavior to me based on its current search path.

Closes #4958

bin/elasticsearch

@@ -89,6 +89,7 @@ if [ "x$ES_INCLUDE" = "x" ]; then
                    /usr/local/share/elasticsearch/elasticsearch.in.sh \
                    /opt/elasticsearch/elasticsearch.in.sh \
                    ~/.elasticsearch.in.sh \
+                   $ES_HOME/bin/elasticsearch.in.sh \
                    "`dirname "$0"`"/elasticsearch.in.sh; do
         if [ -r "$include" ]; then
             . "$include"