support for resetting signals to the _system_ default

[Pádraig Brady]

Currently `trap` has no way to set a signal to the system default.

Rather `trap - PIPE` for example sets the signal handling to
the value it have upon entrance to the shell. It's common enough
for systems to disable SIGPIPE, so it would be useful to
support a shell wrapper that would renable standard SIGPIPE handling.

I'm not sure what syntax to use for this, maybe: `trap + PIPE`

Note we've recently added the --default-signal=PIPE option to env(1)
to support this functionality, though it would be useful
and symmetrical to have this supported by the shell also.

thanks,
Pádraig.