Move getaddrinfo from 'posix' into 'nss'

getaddrinfo is an entry point for nss functionality.  This commit moves
it from 'sysdeps/posix' to 'nss', gets rid of the stub in 'posix', and
moves all associated tests as well.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>