seekdir/readdir patch .. Call for Review.

[Julian Elischer]

if you are interested in readdir(3), seekdir(3) and telldir(3) then 
you should look at
   https://reviews.freebsd.org/D2410

this patches around a problem in seekdir() that breaks Samba.
Seekdir(3) will not work as expected when files prior to the point of 
interest in directory have been deleted since the directory was opened.

Windows clients using Samba cause both these things to happen, causing 
the next readdir(3) after the bad seekdir(3) to skip some entries and 
return the wrong file.

Samba only needs to step back a single directory entry in the case 
where it reads an entry and then discovers it can't fit it into the 
buffer it is sending to the windows client. It turns out we can 
reliably cater to Samba's requirement because the "last returned 
element" is always still in memory, so with a little care, we can
set our filepointer back to it safely. (once)

seekdir and readdir (and telldir()) need a complete rewrite along with 
getdirentries() but that is more than a small edit like this.