Move 'rpc' routines from 'inet' into 'nss'

The getrpcby* and getrpcent* routines are entry points for nss
functionality.  This commit moves them from the 'inet' subdirectory to
'nss'.  The Versions entries for these routines along with a test,
located in the 'sunrpc' subdirectory, are also moved into 'nss'.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>