d-l error handling

[jpryzby+d at quoininc.com (Justin Pryzby)]

I just realized the reason for d-l poor error handling:

/usr/share/live-helper/functions/lockfile.sh:
trap "test -f ${FILE} && rm -f ${FILE}; exit 0" 0 1 2 3 9 15

This exit 0 happens in the helper helper so callers don't notice the
error.

My naive though is that the trap should be:
trap 'ret=$?; '"rm -f \"${FILE}\";"' exit $ret' EXIT

IIRC this works around broken shells or ambiguity about the return
value of the shell process after ret (is it the value of $? on
entering the trap, or after running the trap commands?)

Justin