MDEV-15526 SysV init service deployed file '/etc/init.d/mysql' prevents systemctl disable command to work correctly (mariadb|mysql naming support) (debian/ubuntu)

mariadb service is controlled by systemd

Do not install SysV init script on Debian/Ubuntu in /etc/init.d/
put it in /usr/share/mysql in case the user would need it for something.

Use systemctl in pre/post scripts directly
without funky conditionals and wrapper scripts

debian/mariadb-server-10.4.install

@@ -2,6 +2,7 @@ debian/additions/debian-start etc/mysql
 debian/additions/debian-start.inc.sh usr/share/mysql
 debian/additions/echo_stderr usr/share/mysql
 debian/additions/mysqld_safe_syslog.cnf etc/mysql/conf.d
+debian/additions/mysql.init usr/share/mysql
 etc/apparmor.d/usr.sbin.mysqld
 lib/systemd/system/mariadb@bootstrap.service.d/use_galera_new_cluster.conf
 usr/bin/aria_chk

debian/mariadb-server-10.4.postinst

@@ -17,11 +17,7 @@ ERR_LOGGER="logger -p daemon.err -t mariadb-server-$VER.postinst -i"
 set -o pipefail
 
 invoke() {
-  if [ -x /usr/sbin/invoke-rc.d ]; then
-    invoke-rc.d mysql $1
-  else
-    /etc/init.d/mysql $1
-  fi
+  systemctl $1 mysql
 }
 
 case "$1" in

debian/mariadb-server-10.4.postrm

@@ -12,11 +12,7 @@ MYADMIN="/usr/bin/mysqladmin --defaults-file=/etc/mysql/debian.cnf"
 # is running!
 stop_server() {
   set +e
-  if [ -x /usr/sbin/invoke-rc.d ]; then
-    invoke-rc.d mysql stop
-  else
-    /etc/init.d/mysql stop
-  fi
+  systemctl stop mysql
   errno=$?
   set -e
 

debian/mariadb-server-10.4.preinst

@@ -20,25 +20,18 @@ mysql_upgradedir=/var/lib/mysql-upgrade
 # do it himself. No database directories should be removed while the server
 # is running! Another mysqld in e.g. a different chroot is fine for us.
 stop_server() {
-    if [ ! -x /etc/init.d/mysql ]; then return; fi
-
     # Return immediately if there are no mysql processes running
     # as there is no point in trying to shutdown in that case.
     if ! pgrep mysqld > /dev/null; then return; fi
 
     set +e
-    if [ -x /usr/sbin/invoke-rc.d ]; then
-      cmd="invoke-rc.d mysql stop"
-    else
-      cmd="/etc/init.d/mysql stop"
-    fi
-    $cmd
+    systemctl stop mysql
     errno=$?
     set -e
 
     # 0=ok, 100=no init script (fresh install)
     if [ "$errno" != 0 -a "$errno" != 100 ]; then
-      echo "${cmd/ */} returned $errno" 1>&2
+      echo "'systemctl stop mysql' returned $errno" 1>&2
       echo "There is a MySQL server running, but we failed in our attempts to stop it." 1>&2
       echo "Stop it yourself and try again!" 1>&2
       db_stop