gnu: databases: Define postgresql-10.

gnu: databases: Define postgresql-10.

So that it can be used, rather than postgresql. Because the major version of
the package is important, as it relates to the compatability with the data
files used by the service, it's useful to have a stable name to refer to a
package for version 10 of PostgreSQL.

* gnu/packages/databases.scm (postgresql-10): New variable.