posix_fadvsise in base backups

Attached patch adds a simple call to posix_fadvise with
POSIX_FADV_DONTNEED on all the files being read when doing a base
backup, to help the kernel not to trash the filesystem cache.

Seems like a simple enough fix - in fact, I don't remember why I took
it out of the original patch :O

Any reason not to put this in? Is it even safe enough to put into 9.1
(probably not, but maybe?)

-- 
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/