there's no good reason to allow smtpd to execute custom command set by root

there's no good reason to allow smtpd to execute custom command set by root
in a .forward file so disallow custom commands and file reading, only allow
setting forward addresses and users.

as root is no longer allowed to run any MDA but mbox, we can be stricter on
the setup of the MDA process and refuse to exec anything that's not an mbox
dispatcher.

tested by op@ who edited a root envelope to simulate an exploit injecting a
custom command in a root envelope, smtpd refused to exec.

ok millert@ and op@