bin/hadoop script doesn't allow for different memory settings for each daemon type

bin/hadoop assumes that all daemon types ( namenode, datanode, jobtracker, tasktracker ), all use the same memory settings.. (HADOOP_HEAPSIZE).

I propose changes to that script to allow overriding the default memory ( HADOOP_HEAPSIZE ), with daemon specific OPTS (HADOOP_NAMENODE_OPTS, etc ).

Basically at the bottom of the bin/hadoop script, it will check to see if the user has already set "-Xmx" in the HADOOP_OPTS variable.. if so, then it will ignore the JAVA_HEAP_SIZE variable..

as such:

1. run it
   if [[ $HADOOP_OPTS == -Xmx ]]; then
   exec "$JAVA" $HADOOP_OPTS -classpath "$CLASSPATH" $CLASS "$@"
   else
   exec "$JAVA" $JAVA_HEAP_MAX $HADOOP_OPTS -classpath "$CLASSPATH" $CLASS "$@"
   fi

I will attach the file as I have modified it..