* guix/build/syscalls.scm (errno): Do not export.
(syscall->procedure): Change to return a procedure that returns both the
value and errno. Use #:return-errno? where available.
(mount, umount, swapon, swapoff, mkdtemp!, fdatasync, statfs)
(clone, setns, pivot-root, fcntl-flock, network-interface-names)
(network-interface-flags, set-network-interface-flags)
(set-network-interface-address, network-interface-address):
(network-interfaces, tcgetattr, tcsetattr, terminal-window-size): Adjust
accordingly using 'let-values'.
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