These changes are comparable to Percona's modifications in innodb in the
Percona Xtrabackup repository.

- If functions are used in backup as well as in innodb, make them non-static.

- Define IS_XTRABACKUP() macro for special handling of innodb running
  inside backup.

- Extend some functions for backup.
  fil_space_for_table_exists_in_mem() gets additional parameter
  'remove_from_data_dict_if_does_not_exist', for partial backups

  fil_load_single_table_tablespaces() gets an optional parameter predicate
  which tells whether to load tablespace based on database or table name,
  also for partial backups.

  srv_undo_tablespaces_init() gets an optional parameter 'backup_mode'

- Allow single redo log file (for backup "prepare")

- Do not read doublewrite buffer pages in backup, they are outdated

- Add function fil_remove_invalid_table_from_data_dict(), to remove non-existing
   tables from data dictionary in case of partial backups.

- On Windows, fix file share modes when opening tablespaces,
to allow mariabackup to read tablespaces while server is online.

- Avoid access to THDVARs in backup, because innodb plugin is not loaded,
and THDVAR would crash in this case.