Aliens — Admission — Discretionary power of Attorney-General to admit aliens on parole basis pending determination of final immigration status — Scope of power — South Vietnamese children admitted into the United States on parole basis for adoption by United States families — Allegation that some children ineligible for adoption being illegally detained in the United States in violation of their constitutional rights — Application to obtain records containing details of children’s background — Whether justiciable action — The law of the United States