client/file: report EOF when reading at end of file
client/file: report EOF when reading at end of file

Change-Id: I6185d64a5f61fda63bf0999dcb663d08c48e2455
Reviewed-on: https://upspin-review.googlesource.com/6260
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

upspin/upspin,upspin/upspin