Bind more InnoDB parameters directly to MYSQL_SYSVAR and
remove "shadow variables".

innodb_change_buffering: Declare as ENUM, not STRING.

innodb_flush_method: Declare as ENUM, not STRING.

innodb_log_buffer_size: Bind directly to srv_log_buffer_size,
without rounding it to a multiple of innodb_page_size.

LOG_BUFFER_SIZE: Remove.

SysTablespace::normalize_size(): Renamed from normalize().

innodb_init_params(): A new function to initialize and validate
InnoDB startup parameters.

innodb_init(): Renamed from innobase_init(). Invoke innodb_init_params()
before actually trying to start up InnoDB.

srv_start(bool): Renamed from innobase_start_or_create_for_mysql().
Added the input parameter create_new_db.

SRV_ALL_O_DIRECT_FSYNC: Define only for _WIN32.

xb_normalize_init_values(): Merge to innodb_init_param().