Fix spaces in HADOOP_OPTS arguments

Prefix exec by eval in Hadoop bin scripts
Prior to this change, if HADOOP_OPTS contains any arguments that include a
space, the command is not parsed correctly. For example, if
HADOOP_OPTS="... -XX:OnOutOfMemoryError=\"kill -9 %p\" ...", the bin/hadoop
script will fail with the error "Unrecognized option: -9". No amount of clever
escaping of the quotes or spaces in the "kill -9 %p" command will fix this.

The only alternative appears to be to use 'eval'. Switching to use 'eval'
instead of 'exec' also works, but it results in an intermediate bash process
being left alive throughout the entire lifetime of the Java proces being
started. Using 'exec' prefixed by 'eval' as has been done in this commit gets
the best of both worlds, in that options with spaces are parsed correctly, and
you don't end up with an intermediate bash process as the parent of the Java
process.

This is the exact approach that has been taken with Tomcat as well. See:
http://tomcat.10.x6.nabble.com/Using-eval-vs-exec-in-shell-scripts-td2193116.html
https://github.com/apache/tomcat/commit/3445dc3dba7b15f2fff27faef77003215f62e49a
https://github.com/apache/tomcat/commit/83c0aea60f331eb632dcea8e9919d234903e06d1