There are possible relaying problems with quotes in rcpt addresses.
We talked about stripping quotes or refusing addresses with quotes in
smtpd.

This change adds '"' to the list of illegal characters and makes smtpd
refuse such addresses.  (shellcars() returns 1 when verifying sender
and rcpt addresses.)

Let me know if that's not the right way to go and you rather want to
see quotes stripped than rejected.