Solution:
1. Use optional name parameter in thread_t::start for operating
systems that have thread names.
2. Give start_thread() an optional name parameter for the
thread's name. If this parameter is set, it will be appended to "0MQ:".
If not set, "0MQ" will be used as the thread's name.
3. Give epoll the ability to name its thread. Then use this in
io_thread and reaper to name them.