nspawn: optionally run a stub init process as PID 1 This adds a new switch --as-pid2, which allows running commands as PID 2, while a stub init process is run as PID 1. This is useful in order to run arbitrary commands in a container, as PID1's semantics are different from all other processes regarding reaping of unknown children or signal handling.

nspawn: add new --chdir= switch Fixes: #2192

man: fix references to a few external man pages Noticed in pull request #2067.

nspawn: add new --network-veth-extra= switch for defining additional veth links The new switch operates like --network-veth, but may be specified multiple times (to define multiple link pairs) and allows flexible definition of the interface names. This is an independent reimplementation of #1678, but defines different semantics, keeping the behaviour completely independent of --network-veth. It also comes will full hook-up for .nspawn files, and the matching documentation.

doc: correct orthography, word forms and missing/extraneous words

doc: correct punctuation and improve typography in documentation

man: typo fixes

nspawn: add new .nspawn files for container settings .nspawn fiels are simple settings files that may accompany container images and directories and contain settings otherwise passed on the nspawn command line. This provides an efficient way to attach execution data directly to containers.