MDEV-12218 Clean up InnoDB parameter validation

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().

mysql-test/suite/sys_vars/t/innodb_flush_method_func.test

@@ -0,0 +1,26 @@
+--source include/have_innodb.inc
+# Embedded server tests do not support restarting.
+--source include/not_embedded.inc
+
+call mtr.add_suppression("InnoDB: Failed to set .*DIRECT");
+--replace_result unbuffered fsync
+select @@innodb_flush_method;
+
+create table t(a serial) engine=innodb;
+
+# kill and restart
+FLUSH TABLES;
+let $shutdown_timeout= 0;
+let $restart_parameters=--innodb-flush-method=5;
+--source include/restart_mysqld.inc
+select @@innodb_flush_method;
+
+insert into t values(0);
+# kill and restart
+FLUSH TABLES;
+let $shutdown_timeout= 0;
+let $restart_parameters=--innodb-flush-method=0;
+--source include/restart_mysqld.inc
+select @@innodb_flush_method;
+
+drop table t;