The trap builtin silently ignores invalid signal specifications
if the first argument is "-"; e.g.,

    % trap - INT '@#?junk!*' TERM 

succeeds without complaint.  The attached patch changes this so
that an error is reported (which matches the behaviour if the first
argument is not "-").
I've also amended the documentation for trap, which didn't discuss
the invocations "trap -" and "trap SIG1 SIG2 ...".

Regards,
Thorsten Dahlheimer