auth(
bytes,host,number) ->name,data
Find a display's cookie in an Xauthority file
bytes is the file's content. Returns the first MIT-MAGIC-COOKIE-1 entry
for the display: one on host or on any host, for display number or
for every display. Returns nil, for empty auth, when none matches or the
file is cut short. A server with no cookie takes empty auth from a local
client of its own user.