Access readerSharing in a thread safe way

The field .readerSharing can be used from different theads at the same
time.

We define it as _Atomic to have atomic accesses to the field and fix a
TSAN warning:
==================
WARNING: ThreadSanitizer: data race (pid=63123)
  Write of size 4 at 0x5575884d8cb8 by thread T3:
    #0 EHStatusHandlerThread PCSC/src/eventhandler.c:449:41 (pcscd+0xd6f12) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)

  Previous write of size 4 at 0x5575884d8cb8 by thread T8:
    #0 SCardDisconnect PCSC/src/winscard.c:1034:39 (pcscd+0xe4779) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)
    #1 ContextThread PCSC/src/winscard_svc.c:575:16 (pcscd+0xe6507) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)

  As if synchronized via sleep:
    #0 nanosleep <null> (pcscd+0x515ed) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)
    #1 SYS_USleep PCSC/src/sys_unix.c:87:9 (pcscd+0xd6fca) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)
    #2 EHStatusHandlerThread PCSC/src/eventhandler.c (pcscd+0xd6fca)
    #3 __tsan_thread_start_func <null> (pcscd+0x53d66) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)

  Location is global 'readerStates' of size 2944 at 0x5575884d8c30 (pcscd+0x1497cb8)

  Thread T3 (tid=63127, running) created by main thread at:
    #0 pthread_create <null> (pcscd+0x53dfd) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)
    #1 ThreadCreate PCSC/src/utils.c:184:8 (pcscd+0xe3cab) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)
    #2 EHSpawnEventHandler PCSC/src/eventhandler.c:233:7 (pcscd+0xd6840) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)
    #3 RFAddReader PCSC/src/readerfactory.c:397:8 (pcscd+0xdb6dd) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)
    #4 HPAddDevice PCSC/src/hotplug_libudev.c:512:8 (pcscd+0xe3409) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)
    #5 HPScanUSB PCSC/src/hotplug_libudev.c:579:3 (pcscd+0xe2a1d) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)
    #6 HPRegisterForHotplugEvents PCSC/src/hotplug_libudev.c:761:2 (pcscd+0xe2a1d)
    #7 main PCSC/src/pcscdaemon.c:768:7 (pcscd+0xd8717) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)

  Thread T8 (tid=63221, finished) created by main thread at:
    #0 pthread_create <null> (pcscd+0x53dfd) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)
    #1 ThreadCreate PCSC/src/utils.c:184:8 (pcscd+0xe3cab) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)
    #2 CreateContextThread PCSC/src/winscard_svc.c:256:7 (pcscd+0xd924e) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)
    #3 SVCServiceRunLoop PCSC/src/pcscdaemon.c:133:9 (pcscd+0xd924e)
    #4 main PCSC/src/pcscdaemon.c:801:2 (pcscd+0xd8810) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)
    #5 main PCSC/src/pcscdaemon.c:801:2 (pcscd+0xd8810) (BuildId: b2f519c8807f458b6282d631fe17086d4f6da420)

SUMMARY: ThreadSanitizer: data race PCSC/src/eventhandler.c:449:41 in EHStatusHandlerThread
==================

src/eventhandler.h

@@ -54,7 +54,7 @@ THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  char readerName[MAX_READERNAME]; /**< reader name */
  uint32_t eventCounter; /**< number of card events */
  uint32_t readerState; /**< SCARD_* bit field */
- int32_t readerSharing; /**< PCSCLITE_SHARING_* sharing status */
+ _Atomic int32_t readerSharing; /**< PCSCLITE_SHARING_* sharing status */
 
  UCHAR cardAtr[MAX_ATR_SIZE]; /**< ATR */
  _Atomic uint32_t cardAtrLength; /**< ATR length */