* gnu/services/base.scm (<network-link>): Add mac-address field. Set
type field to #f by default, so it won't be mandatory. network-link
without a type will be used for existing interfaces.
(assert-network-link-mac-address, mac-address?): Add sanitizer. Allow
valid mac-address or #f.
(assert-network-link-type): Add sanitizer. Allow symbol or #f.
* gnu/services/base.scm (network-set-up/linux,
network-tear-down/linux): Adapt to new structure.
* doc/guix.texi (Networking Setup): Document it.
* gnu/tests/networking.scm (run-static-networking-advanced-test): New
variable.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>