i noticed that i couldn't import /net.alt from a server.
on the outside network it was in the name space from /lib/namespace.ext.
exportfs's newns used the default.  fair enough but surprisingly
a compensating bind /lib/namespace.ext /lib/namespace
in /bin/service.alt/il17007 didn't work, because exportfs does two
newns!  auth_chuid does one itself, which uses the
rebound /lib/namespace, but leaves a name space in which
the bind no longer was there, and then on return exportfs
does another, using the `original' /lib/namespace,
which doesn't set up (the right) /net.alt.

two changes: one, remove the second newns, because it didn't seem
justified.  two, add a -n option to allow a name space file to
be specified explicitly (which is perhaps more straightforward
in some cases than the bind).

i added an explanatory remark about name space creation
to the manual page, and in that part i documented the
new -n option.