[#521782] unreliable file.read() error handling

* Objects/fileobject.c
  (file_read): Clear errors before leaving the loop in all situations,
  and also check if some data was read before exiting the loop with an
  EWOULDBLOCK exception.

* Doc/lib/libstdtypes.tex
* Objects/fileobject.c
  Document that sometimes a read() operation can return less data than
  what the user asked, if running in non-blocking mode.

* Misc/NEWS
  Document the fix.