Based on discussions with Rohan Prinja <rohan.prinja@gmail.com>.
* guix/build/syscalls.scm (<interface>): New record type.
(write-interface, values->interface, unfold-interface-list,
network-interfaces, free-ifaddrs): New procedures.
(ifaddrs): New C struct.
(%struct-ifaddrs-type, %sizeof-ifaddrs): New macros.
* tests/syscalls.scm ("network-interfaces returns one or more interfaces",
"network-interfaces returns \"lo\""): New tests.
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