Revision 1884077

apr_thread: destroy the thread's pool at _join() time, unless _detach()ed.

Destroying a joinable thread pool from apr_thread_exit() or when the thread
function returns, i.e. from inside the thread itself, is racy or deadlocky
with APR_POOL_DEBUG, with the parent pool being destroyed.

This commit adds a ->detached flag in each arch's apr_thread_t struct to track
whether a thread is detached (either at _create() or _detach() time). If
detached, the pool is destroyed when the thread exits, otherwise when the
thread is joined with apr_thread_join().