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.

contrib/packaging/builddeb

@@ -13,6 +13,13 b' CLEANUP=1'
 DISTID=`(lsb_release -is 2> /dev/null | tr '[:upper:]' '[:lower:]') || echo debian`
 CODENAME=`lsb_release -cs 2> /dev/null || echo unknown`
 DEBFLAGS=-b
+
+cleanup() {
+    if [ '$CLEANUP' ]; then
+        rm -r '$PWD/debian';
+    fi
+}
+
 while [ "$1" ]; do
     case "$1" in
     --distid )
@@ -44,7 +51,7 b' while [ "$1" ]; do'
     esac
 done
 
-trap "if [ '$CLEANUP' ] ; then rm -r '$PWD/debian' ; fi" EXIT
+trap 'cleanup' EXIT
 
 set -u