MDEV-8576: Bootstrap should ignore --enforce-storage-engine option

Allow enforce-storage-engine="" option and use that on mysql_install_db
when doing bootstrap.

scripts/mysql_install_db.pl.in

@@ -428,6 +428,7 @@ my $mysqld_install_cmd_line = quote_options($mysqld_bootstrap,
                                             "--max_allowed_packet=8M",
                                             "--default-storage-engine=MyISAM",
                                             "--net_buffer_length=16K",
+                                            "--enforce-storage-engine=\"\"",
                                             @args,
                                           );
 

scripts/mysql_install_db.sh

@@ -417,7 +417,7 @@ mysqld_bootstrap="${MYSQLD_BOOTSTRAP-$mysqld}"
 mysqld_install_cmd_line()
 {
   "$mysqld_bootstrap" $defaults "$mysqld_opt" --bootstrap \
-  "--basedir=$basedir" "--datadir=$ldata" --log-warnings=0 \
+  "--basedir=$basedir" "--datadir=$ldata" --log-warnings=0 --enforce-storage-engine="" \
   $args --max_allowed_packet=8M \
   --net_buffer_length=16K
 }

sql/mysqld.cc

@@ -5293,6 +5293,9 @@ static int init_server_components()
   if (default_tmp_storage_engine && !*default_tmp_storage_engine)
     default_tmp_storage_engine= NULL;
 
+  if (enforced_storage_engine && !*enforced_storage_engine)
+    enforced_storage_engine= NULL;
+
   if (init_default_storage_engine(default_tmp_storage_engine, tmp_table_plugin))
     unireg_abort(1);