Update skarnet.org software for 2.0 release.

Major change is new build system using configure script and GNU Make
4.0.

execline           2.0.0.0
s6                 2.0.0.0
s6-dns             2.0.0.0
s6-linux-utils     2.0.0.0
s6-networking      2.0.0.0
s6-portable-utils  2.0.0.0
skalibs            2.0.0.0