services: nginx: Add support for 'location' blocks.

* gnu/services/web.scm (<nginx-server-configuration>): Add field 'locations'.
(<nginx-location-configuration>): New record type.
(<nginx-named-location-configuration>): New record type.
(nginx-location-config): New function.
(default-nginx-server-config): Include locations.
* doc/guix.texi (Web Services): Document the new nginx-location-configuration
and nginx-named-location-configuration data types, as well as the changes to
the nginx-server-configuration.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>