update to latest version of skarnet.org software

Major skalibs release, so all the updates must be done at the same time. This branch updates: skalibs, utmps, execline, s6, s6-linux-init, s6-rc, s6-dns, s6-networking, s6-portable-utils, s6-linux-utils, bncm, mdevd. Also makes sure sysvinit invokes s6 with the correct options, when s6 runs under sysvinit. The individuals commit are a mess because I'm an idiot, but in the end everything should be working.