fix thread leak on timer_create(SIGEV_THREAD) failure

fix thread leak on timer_create(SIGEV_THREAD) failure

After commit 5b74eed3b301e2227385f3bf26d3bb7c2d822cf8 the timer thread
doesn't check whether timer_create() actually created the timer,
proceeding to wait for a signal that might never arrive.  We can't fix
this by simply checking for a negative timer_id after
pthread_barrier_wait() because we have no way to distinguish a timer
creation failure and a request to delete a timer with INT_MAX id if it
happens to arrive quickly (a variation of this bug existed before
5b74eed3b301e2227385f3bf26d3bb7c2d822cf8, where the timer would be
leaked in this case).  So (ab)use cancel field of pthread_t instead.