Make /{bin,lib,share,include} a symlink to /usr/{bin,lib,share,include}

See merge request redox-os/redox!1400

This matches the directory layout of modern Linux systems.

Move /etc/group generation to the installer

See merge request redox-os/redox!1401

This ensures that the gid's for the auto-generated user groups stay in
sync with the gid in /etc/passwd. It also makes it easier to evolve the
format of /etc/group in the future.

add /dev/tty and other libc: links

See merge request redox-os/redox!1399

Use the new config include functionality for creating the base system files

See merge request redox-os/redox!1398

Use sparse allocation of extra.img on Linux too

See merge request redox-os/redox!1391