POSIX recommendations.
- trap now accepts signal names and signal numbers
e.g. INT, SIGINT, 2
- added option -l that outputs a list of valid signals
- added signal EXIT to list of valid signals
- a `-' in the action part will reset specified signal to their
default behaviour
- changed standard output format to make it suitable as an input
to another shell that achieves the same trapping results
8d23cb0373