nvmeprint.cpp: Fix self-test for single namespace devices.

Use broadcast NSID if only one namespace is supported
(#1741, GH issues/217).
smartctl.8.in: Document new behavior.

git-svn-id: https://svn.code.sf.net/p/smartmontools/code/trunk@5603 4ea69e1a-61f1-4043-bf83-b5c94c648137