nsenter: Rename --network option to --net

nsenter: Rename --network option to --net

In nsenter from util-linux, the long version of the -n option is
--net=<path>.   BusyBox's version expects --network=<path>, so scripts
and examples written for util-linux's version cause BusyBox's version
to exit with the usage message.

Confusingly, until commit 036585a911a5f, the usage message erroneously
claimed that the long option was indeed called --net; after that commit
long options are not listed at all.

Signed-off-by: Euan Harris <euan.harris@docker.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>