This allows more realistic testing, with the "outside" copy of secnet
in a separate environment with its own instance of the network stack.
We have to go through some contortions to get the user a shell in the
"outside" environment, since unshare -n also breaks AF_UNIX, and we
want to keep the terminal secnet is invoked in just for secnet.
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>