recv_sys_t::parse(): Correctly handle the storing==BACKUP case,
and simplify some logic around storing==YES as well.
The added test mariabackup.undo_truncate is based on an idea of
Thirunarayanan Balathandayuthapani. It nondeterministically (not on
every run) covers this logic, including the function backup_undo_trunc(),
for both innodb_encrypt_log=ON and innodb_encrypt_log=OFF.
Reviewed by: Debarun Banerjee
