Fix race condition with respects to cleaning up the profiling threads

when in use.

The problem here is the polling threads in the various acct_gather codes
were detached and could possibly still be polling after the plugin had
been unloaded making a seg fault with a backtrace like this...

#0  0x00007fe7af008c00 in ?? ()
#1  0x00007fe7b1138479 in __nptl_deallocate_tsd () at pthread_create.c:175
#2  0x00007fe7b11398b0 in __nptl_deallocate_tsd () at pthread_create.c:326
#3  start_thread (arg=0x7fe7b1f12700) at pthread_create.c:346
#4  0x00007fe7b0e6fb5d in clone ()
    at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109

The fix was to make the threads non-detached and join them before calling
a dlclose.