Implement innodb_flush_method as an enum parameter in Mariabackup,
instead of ignoring the option and hard-wiring it to a default value.

xb0xb.h: Remove. Only xtrabackup.cc refers to the enum parameters.

innodb_flush_method_names[], innodb_flush_method_typelib[]:
Define as non-static, so that mariabackup can share the definitions.

srv_file_flush_method: Change the type to ulong, to match the
assignment in init_one_value() and handle_options() in mariabackup.