EXIT trap doesn't trigger when exiting from other signals

[Crestwave]

Used in #679. The great thing about Bash's trap is that you can trap EXIT and it will trigger on all other kinds of signals that make the program exit. It's a bit (very if you want to make it compatible with various POSIX shells) complicated to handle cleanup functions correctly without this, as simply trapping INT will prevent it from exiting on INT.

$ cat <<'EOF' >a
> trap 'echo foo' EXIT
> sleep 999
> EOF
$ bash a
^Cfoo

$ osh a
^C
$ 

[andychu]

Related: #619 for DEBUG trap