[Bug-tar] Use posix_fadvise to improve archive creation performance

[Carlo Alberto Ferraris]

This is a PoC patch that improves archive creation performance at least in 
certain configurations (ubuntu x64 14.04, virtualized infrastructure, SAN 
storage) by using posix_fadvise to signal the intent to sequentially read the 
whole file. 

gnulib currently does not provide posix_fadvise. On platforms where 
posix_fadvise is not available (e.g. my macbook) this patch silently reverts to 
a noop. 

Carlo

0001-use-posix_fadvise-to-prefetch-files-when-creating-ar.patch
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