MDEV-19933: Sync mariadb-common and update-alternatives based /etc/mysql/

Split the big my.cnf into multiple smaller files with the same filenames
and contents as official Debian/Ubuntu packaging has.

The config contents stays the same apart from following additions
which the original MariaDB upstream configs had and probably needs
to be kept:
- lc-messages=en_US and skip-external-locking in server config

Configs the original MariaDB upstream had that are seemingly
unnecessary and thus removed:
- port=3306 removed from the client config
- log_warnings=2 removed from server config

Also adopt update-alternatives system using
mysql-common/configure-symlinks. This way it is aligned with
downstream Debian/Ubuntu packaging.

debian/mariadb-common.postrm

@@ -1,9 +1,16 @@
-#!/bin/bash
+#!/bin/sh
+
 set -e
 
-if [ "$1" = "purge" ]; then
-  rmdir /etc/mysql/conf.d 2>/dev/null || true
-  rmdir /etc/mysql 2>/dev/null || true
-fi
+case "$1" in
+  remove|disappear)
+    # New packaging paradigm for my.cnf handling among MySQL variants
+    # Used in Ubuntu since Dec-2014 and in Debian since Jul-2015
+    if [ -f /usr/share/mysql-common/configure-symlinks ]
+    then
+      /usr/share/mysql-common/configure-symlinks remove mariadb "/etc/mysql/mariadb.cnf"
+    fi
+  ;;
+esac
 
 #DEBHELPER#