Remove FileSystemUtils#maxOpenFiles

This method basically forcefully creates as many files as possible
to find out the process limit in a brute-force manner. The number of
possible probles with this approach would exceed the number of lines
left on this commit message.

This commit uses a JMX based alternative to print the process limit.
Cleanup FileSystemUtils#mkdirs(File)

This methods had some workarounds for bugs that seem to be fixed
in Java 7 [1]. There seem to be other problems on shared file-systems
which are not really supported by lucene anyway or rather not
recommeded. Yet the current solution that interrupts a static thread
reference is too dangrous given all the usage of NIO across
elasticsearch.

[1] http://bugs.java.com/bugdatabase/view_bug.do?bug_id=4742723