sysusers: always treat ENOENT as entry-not-found when doing NSS calls

sysusers: always treat ENOENT as entry-not-found when doing NSS calls

For most NSS calls it is documented that they return NULL + errno=0 when
an entry is not found. However, in reality it appears to be common to
return NULL + errno=ENOENT, instead. Handle that correctly, and don't
consider ENOENT a systematic error.