Fix package deinstallation: only move X.bck over X if the former exists.
Since pkg_deinstall(1) invokes the package deinstall script twice,
at the second invocation X.bck is no longer there, and mv(1), even with
the -f flag, returns a non-zero exit code. The shell executing the script
propagates this exit code right back unto pkg_deinstall(1), which complains
and aborts the package deinstallation.
I wish I had not forgotten about this problem, so this could go in
before DP1 was cut :(
Approved by: no response from trevor@, the port maintainer