DAEMON_EXAMPLE PLUGIN

The deinit() function of the daemon example plugin was 
calling pthread_cancel, but not waiting for the worker 
thread to actually complete before deallocating the data
buffer and closing the file that it writes to.
Fixed by adding a pthread_join to wait for the thread 
to complete before doing the cleanup work that may 
affect it.
Test case added.
Removed a stray 'x' variable from the example code.