David Laight reports that sometimes reads on a serial port will

David Laight reports that sometimes reads on a serial port will
attempt to block, and hence return EAGAIN/EWOULDBLOCK, in spite of
the port having been reported readable by select(2). Don't treat
those errors as fatal.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9020 cda61777-01e9-0310-a592-d414129be87e