Deb: make libmariadb3 to provide the libmariadbclient.so.18 link

In Debian many existing applications in Debian/Ubuntu have been built
while libmariadbclient-dev or libmariadbclient-dev-compat was installed.

To satisfy installation dependencies, the package name libmariadbclient18
must be provided, and at runtime a shared library must by the name
libmariadbclient.so.18.

Provide these to remain backwards compatible.

debian/libmariadb3.install

 usr/lib/mariadb/libmariadb.so.*
+usr/lib/*/libmariadbclient.so.*
 usr/lib/mysql/plugin/dialog.so
 usr/lib/mysql/plugin/mysql_clear_password.so

debian/rules

@@ -157,6 +157,9 @@ override_dh_auto_install:
 	# install Apport hook
 	install -D -m 644 debian/mariadb-server-10.2.py $(TMP)/usr/share/apport/package-hooks/source_mariadb-10.2.py
 
+	# Install libmariadbclient18 compatibility links
+	ln -s /usr/lib/$(DEB_HOST_MULTIARCH)/libmariadb.so.3 $(TMP)/usr/lib/$(DEB_HOST_MULTIARCH)/libmariadbclient.so.18
+
 	touch $@
 
 override_dh_installlogrotate-arch: