fixes, clarifications, cleanups for RTDM

Hi,

this patch
o fixes a cleanup-on-error problem with rtdm_task_init
o moves this call into drvlib (i.e. de-inlines it)
o gets rid of __u64 and __s64 usages in favour of uint64_t and int64_t (for consistent types in both kernel and user space)
o tries to improve the documentation of rtdm_device.device_name and RTDM_EXECUTE_ATOMICALLY and
o moves EXPORT_SYMBOLs to their related functions

Tested, caused no regressions so far. Please apply.

Jan