tfork: add a suppresssions file for helgrind

tfork: add a suppresssions file for helgrind

tfork_atexit_unknown[1|2]:

  No idea what triggers this, definitely not tfork itself.

tfork_pthread_get_specific:

 Helgrind reports:

 Possible data race during read of size 4 at 0x5141304 by thread #3
 Locks held: none
    at 0x50E602E: tfork_global_get (tfork.c:301)
    by 0x50E69B1: tfork_create (tfork.c:737)
    by 0x2F7419: tfork_thread (tfork.c:431)
    by 0x4C35AC5: mythread_wrapper (hg_intercepts.c:389)
    by 0x8D38593: start_thread (pthread_create.c:463)
    by 0x9A70E6E: clone (clone.S:95)

 This conflicts with a previous write of size 4 by thread #2
 Locks held: none
    at 0x8D3F7B7: pthread_key_create (pthread_key_create.c:41)
    by 0x50E5F79: tfork_global_initialize (tfork.c:280)
    by 0x8D3FEA6: __pthread_once_slow (pthread_once.c:116)
    by 0x50E6999: tfork_create (tfork.c:728)
    by 0x2F7419: tfork_thread (tfork.c:431)
    by 0x4C35AC5: mythread_wrapper (hg_intercepts.c:389)
    by 0x8D38593: start_thread (pthread_create.c:463)
    by 0x9A70E6E: clone (clone.S:95)
  Location 0x5141304 is 0 bytes inside global var "tfork_global_key"
  declared at tfork.c:122

  This is nonsense, tfork_global_get() calls pthread_getspecific, so
  we're looking at the pthread_key_create()/pthread_[g|s]etspecific()
  API here which works with threads by design.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>