Deb: rename client library packages to reflect its contents

The new library name libmariadb3 matches the libmariadb.so.3 filename.
Packages that want to build against MariaDB Connector C have as
build-dependency libmariadb-dev and as run-time dependency libmariadb3.

Make the package provide libmariadbclient18 for backwards compatibility,
though compatibility is not always assured. There library did change
to a whole new generation after all, even though ABI compatibility has
been a design goal.

Also do the equivalent change for the -dev package. Packages that
explicitly want to use the MariaDB Connector C should mark it as
their build-depends.

Also provide an empty libmariadbclient18 metapackage to facilitate
upgrades from old MariaDB installs, just like there is an empty
libmysqlclient18 package. Create more of these in the future as needed.

debian/libmariadb-dev.README.Maintainer

@@ -0,0 +1,3 @@
+The examples directory includes files that might be needed by some
+developers:
+- the example file udf_example.c