usb_device.c: Removed libusb_exit from all functions but USB_Close (otherwise crash).
p11generic.h: There was a re-entering of a mutex (in C_CloseAllSessions) which is legal in windows, but blocks in pthreads.
New macros LOCKED_UNLOCK and LOCKED_RELOCK introduced for giving up and reestablish the lock (used in C_CloseAllSessions).
Changes the thread-safe macros slightly (not using goto anymore, some guys hat them ...).