services: network-manager: Use record for configuration.

* gnu/services/network-manager.scm (<network-manager-configuration>): New
record type.
(network-manager-shpeherd-service): Change to use the
network-manager-configuration record, rather than a package.  Generate a
simple configuration file from the network-manager-configuration record.
(network-manager-service-type): Update extensions to take the
network-manager-configuration rather than a package.
(network-manager-service): Remove function, the network-manager-service-type
can be used instead, and this avoids keeping the function signature and value
coresponding to the service type in sync.
* doc/guix.texi (Networking Services): Remove documentation for the removed
network-manager-service procedure, and add documentation of the
network-manager-service-type variable and network-manager-configuration
record.

Signed-off-by: 宋文武 <iyzsong@member.fsf.org>