Fix rsync and thread pool bugs.

Fix rsync and thread pool bugs.

+Mistakenly (of course, it was a bug) the returned value from rsync execution was being confused with the returned value from execvp call. The main problem was when rsync returned a code 12 (Error in rsync protocol data stream); in that case, the caller confused that error with ENOMEM (also with value 12), which led to terminate execution.
+The thread pool wait function wasn't considering pending taks at the queue; also the poll function was holding and releasing the mutex more than it was needed, and the thread attributes are now globally initialized (thanks @ydahhrk for the code review).
+Increment the number of threads at the internal pool to 10.