Add line numbers and function names to debug output

Add line numbers and function names to debug output by setting the PS4 environment variable.

Please change line 4482-4483 of abcde like this:
From
```
		D) set -x ;;
		h) usage; exit ;;
```
to this
```
		D) set -x
           PS4='+ ${LINENO}(${FUNCNAME}): '
           ;;
		h) usage; exit ;;
```

These changes were tested while working on issue #29 and add very helpful context to the debug output.
