Move getnameinfo from 'inet' to 'nss'

getnameinfo is an entry points for nss functionality.  This commit moves
it from the 'inet' subdirectory to 'nss'.  The corresponding Versions
entry is also moved from 'posix' into 'nss'.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>