r18319: fixed the directory search resume code on IRIX

r18319: fixed the directory search resume code on IRIX

The problem was twofold:

  1) irix returns 64 bit numbers in telldir(). The protocol uses a 32
     bit resume key. We now cope with this properly using the code in
     pvfs_list_seek_ofs().

  2) irix returns 0xFFFFFFFF from telldir() for the last entry in the
     directory. When added to DIR_OFFSET_BASE this became
     DIR_OFFSET_DOTDOT which meant an infinite loop!
(This used to be commit 8cce9740ed0da9f08d6821beb4acaa9d28d149c2)