COMMON: Populate nobody uid/gids after reading from passwd
Since nobody gid/uids vary across OSes, read from passwd and populate these at startup. This assumes that Mapping::Init is called before other Vid calls, at MGM & FUSEX startup this is indeed the case
Fixes: EOS-5512 Signed-off-by: Abhishek Lekshmanan abhishek.lekshmanan@cern.ch