[merge][#279381] correct calls to readdir from pyrex

This fixes https://bugs.edge.launchpad.net/bzr/+bug/279381 plus one
nearby bonus bug.

If the system call is interrupted with eintr or eagain, the previous
code would incorrectly assume that it was at the end of the directory,
but in fact it should continue reading.  It's probably unlikely this
would be hit, but if it was then it might cause us to think some files
had disappeared.  (There might have been a bug report of this.)

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-- 
Martin <http://launchpad.net/~mbp/>