packaging: blindly factor out trap's cleanup function in builddeb This commit blindly extracts builddeb's trap routine in a dedicated function. While doing so, I think two bugs are exposed, which will be addressed in the next commits: - single quoting around '$CLEANUP' will always evaluate to the literal '$CLEANUP' regardless of the variable's value. The "if" will always be true. - the removal operation will not expand $PWD (and a variable expansion would need double quotes, anyways.