Update versions of skarnet.org packages

skalibs            2.3.2.0
execline           2.1.1.0
s6                 2.1.3.0
st-portable-utils  2.0.4.0
s6-linux-utils     2.0.2.0
s6-networking      2.1.0.0

Also change path to s6 static library in s6-networking build.