nixos/network-interfaces: Sanitize sys-subsystem device names

Currently, nixos will allow for interface names with special characters
such as the hyphen to be used. This presents a problem when using
systemd device names as the namespace paths are separated using hyphens.
Within systemd, if a device name has a hyphen it should be replaced with
the escape sequence \x2d.

This patch sanitizes all interface names before they are used in a
systemd device string.