New package: mariadb-c-client (#7729)

* mariadb-c-client is a new package using the distinct, LGPL,
  MySQL-compatible client library from mariadb.com. It provides the
  virtual package mariadb-client
* mariadb is recorded as a provider of the mariadb-client virtual
* The mysql-client virtual package is also added, and mariadb-c-client
  is recorded as a provider for it

etc/spack/defaults/packages.yaml

@@ -30,9 +30,11 @@ packages:
       java: [jdk, openjdk, ibm-java]
       jpeg: [libjpeg-turbo, libjpeg]
       lapack: [openblas]
+      mariadb-client: [mariadb-c-client, mariadb]
       mkl: [intel-mkl]
       mpe: [mpe2]
       mpi: [openmpi, mpich]
+      mysql-client: [mariadb-c-client]
       opencl: [pocl]
       openfoam: [openfoam-com, openfoam-org, foam-extend]
       pil: [py-pillow]