update skarnet.org packages

execline:          2.1.4.5 -> 2.2.0.0
s6-dns:            2.0.0.7 -> 2.1.0.0
s6-linux-utils:    2.0.2.3 -> 2.2.0.0
s6-networking:     2.1.0.4 -> 2.2.1.0
s6-portable-utils: 2.1.0.0 -> 2.1.0.0 (no version change)
s6-rc:             0.0.2.1 -> 0.1.0.0
s6:                2.2.4.3 -> 2.4.0.0
skalibs:           2.3.9.0 -> 2.4.0.1

Also use new --enable-absolute-paths configure arg to correctly set
paths to runtime executables to point within the nix store rather than
relying on PATH resolution.