[MAN] 0 and EXIT both work for exit traps

I saw a discussion in which there was some confusion over whether or not
you can use a symbolic name, since the manpage doesn't specifically say so.

Signed-off-by: Adam Buchbinder <adam.buchbinder@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>