wrap every unlink call to check for != /dev/null

Unlinking /dev/null is bad, we shouldn't do that. Also, we should print
at least a warning if we tried to unlink a file but didn't manage to
pull it of (ignoring the case were the file is /dev/null or doesn't
exist in the first place).

This got triggered by a relatively unlikely to cause problem in
pkgAcquire::Worker::PrepareFiles which would while temporary
uncompressed files (which are set to keep compressed) figure out that to
files are the same and prepare for sharing by deleting them. Bad move.
That also shows why not printing a warning is a bad idea as this hide
the error for in non-root test runs.

Git-Dch: Ignore

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