Fix bug that not handling EOF correctly in |thread()| and |search_read()|.

This commit also switch API from |get_record_keep()| to |get_records_keep()|.
Note that |rk| < 0 implies |fd| < 0, there is no need to check |fd|.


git-svn-id: http://opensvn.csie.org/pttbbs/trunk@6011 63ad8ddf-47c3-0310-b6dd-a9e9d9715204