* gnu/services/databases.scm (postgresql-configuration-log-directory): New
procedure.
(<postgresql-configuration>)[log-directory]: New field.
(postgresql-activation): Create the log directory.
(postgresql-shepherd-service): Honor it.
* gnu/tests/databases.scm (%postgresql-log-directory): New variable.
(log-file): New test case.
* doc/guix.texi (Database Services): Document it.