Rename libmysqclient files and sonames to libmariadbclient

While packaging MariaDB for official Debian inclusion, one of the most invasive changes I made was to rename the shared library filenames in libmysqlclient.so & co to libmariadbclient.so.

The rationale (originally presented by Debian MySQL packagers team) was that as the MySQL and MariaDB libraries are not 100% identical, they should not have the same filename (or soname), otherwise there will be ungraceful failures when a software optimized for MariaDB is run in an environment with the non-MariaDB version of soname libmysqlclient.

In current official Debian packages (source at http://anonscm.debian.org/gitweb/?p=pkg-mysql/mariadb-5.5.git and -10.0.git) the packages libmariadbclient18 and libmysqlclient18 are co-installable and the filenames are not the same (libmariadbclient.so.18 and libmysqlclient.so.18).

It seems that the original MySQL version of libmysqlclient.so.18 can be used with the MariaDB client/server. Therefore software that link to libmysqlclient.so.18 will work just fine with MySQL libmysqlclient.so.18.so and MariaDB server/client. Also if any software was to link to libmariadbclient.so.18 if would work via MariaDB libmysqlclient.so.18.so and MariaDB server/client just fine.