xtrabackup_copy_log(), xtrabackup_copy_logfile():
Change the Boolean parameter to an enum, with the values
COPY_FIRST, COPY_ONLINE, COPY_LAST.

xtrabackup_copy_log(): Return the latest scanned LSN,
which may be less than the last copied LSN. Remove some
dead code that was duplicating some logic that in 10.2
has been moved to log_group_read_log_seg().

log_copying_thread(): Correct the termination condition.

stop_backup_threads(): Shut down the threads that were
created during backup.