Update skarnet.org software for 2.0 release.

New build system using configure script and GNU Make 4.0, and new
releases of the following using the new build system:

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