Avoid fadvise (since it doesn't work as expected)

Avoid fadvise (since it doesn't work as expected)

Currently, it appears that at least on Linux posix_fadvise() always
purges the cache given POSIX_FADV_DONTNEED, which is what bup uses, and
does nothing for POSIX_FADV_NOREUSE, which means that before this patch,
a bup save would completely clear the filesystem cache of any file data
traversed during the run.  Aside from being completely unintended, this
meant that active VM images, large databases, etc. would probably be
purged with every save.

Since it also looks like at least NetBSD doesn't do anything for
DONTNEED, and tools like tar, cpio, and duplicity don't use it, let's
just drop it.  That's simpler, and we can always add it back if/when
someone discovers it helps somewhere relevant.

Thanks to Nimen Nacnamme for reporting the issue.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>
Acked-by: Greg Troxel <gdt@lexort.com>