Remove 'gshadow' and merge into 'nss'

The majority of gshadow routines are entry points for nss functionality.
This commit removes the 'gshadow' subdirectory and moves all
functionality and tests to 'nss'.  References to gshadow/ are
accordingly changed.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>