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.