services: postgresql: Add socket directory support.

* gnu/services/databases.scm (postgresql-config-file-socket-directory): New
procedure.
(<postgresql-config-file>)[socket-directory]: New field.
(postgresql-config-file-compiler): Honor it.
(postgresql-activation): Create the socket directory if needed.
* doc/guix.texi (Database Services): Document it.
* gnu/tests/guix.scm (%guix-data-service-os): Adapt it.
* gnu/tests/monitoring.scm (%zabbix-os): Ditto.
* gnu/tests/web.scm (patchwork-os): Ditto.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>