network-interfaces: Add support for creating vlans

This patch adds support for vlan creation at machine startup, so that we
can adjust interface settings for vlan devices using nix.
Merge pull request #1431 from wkennington/vlan

network-interfaces: Add support for creating vlans
network-interfaces: Add mtu setting support

Adds a setting to the network interface configuration for changing the
mtu for each specified interface.
Merge pull request #1430 from wkennington/mtu

network-interfaces: Add mtu setting support
Use CLisp 2.44.1 for SBCL build --- some releases like it better for bootstrapping...