A merge error caused InnoDB bootstrap to fail when
innodb_undo_tablespaces was set to more than 2.
This was because of a bug that was introduced to
srv_undo_tablespaces_init() by the merge.

Furthermore, some adjustments for Oracle Bug#25551311 aka
Bug#23517560 changes were forgotten. We must minimize direct
references to srv_undo_tablespaces_open and use predicates
instead.

srv_undo_tablespaces_init(): Increment srv_undo_tablespaces_open
once, not twice, per loop iteration.

is_system_or_undo_tablespace(): Remove (unused function).

is_predefined_tablespace(): Invoke srv_is_undo_tablespace().