fix hang because of infinite loop reading EOF

Read() should return empty string when EOF happen, instead of looping
forever. This is the right semantics of read(), and a wrapped version
should not change it.

If you read the read(2) system call manpage, it tells you that when EOF
is seen, return value is 0; it does not say

	``loop forever when EOF happen''.

After the EOF detection is patched you can see the
following exception:

  WARNING: ERROR attempting to copy message 344 for account Gmail:Traceback (most recent call last):
    File "/usr/lib/pymodules/python2.6/offlineimap/folder/Base.py", line 282, in copymessageto
      message = self.getmessage(uid)
    File "/usr/lib/pymodules/python2.6/offlineimap/folder/IMAP.py", line 216, in getmessage
      initialresult = imapobj.uid('fetch', '%d' % uid, '(BODY.PEEK[])')
    File "/usr/lib/python2.6/imaplib.py", line 753, in uid
      typ, dat = self._simple_command(name, command, *args)
    File "/usr/lib/python2.6/imaplib.py", line 1060, in _simple_command
      return self._command_complete(name, self._command(name, *args))
    File "/usr/lib/python2.6/imaplib.py", line 890, in _command_complete
      raise self.abort('command: %s => %s' % (name, val))
  abort: command: UID => socket error: EOF

Signed-off-by: Bao Haojun <baohaojun@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

Changelog.draft.rst

@@ -19,6 +19,7 @@ Changes
 Bug Fixes
 ---------
 
+* Fix hang because of infinite loop reading EOF.
 * Fix regression (UIBase is no more).
 
 * Make profiling mode really enforce single-threading