Factor out printf-style escape parsing logic from echo.c.
unshare: Fix help and option parsing

The help text was inconsistent, and option parsing was completely broken
(the options mostly did the wrong thing).
nsenter: A tool to use setns(2)

This implements all of the namespace parts of nsenter, but UID and GID
switching are missing, as are -r and -w (both because they're not strictly
necessary and because the nsenter manpage has an insufficient
description of how they work).
Add TOYBOX_NORECURSE so xexec() won't make internal function calls.
More static analysis fixes from Ashwini Sharma.
Random in-progress snapshot of sed, not finished yet.