[Bro] postfix instead of sendmail for bro emails

[Gary Faulkner]

I seem to recall a conversation about substituting postfix for sendmail 
for sending email from bro/broctl, but now I can't find it. Is there 
anything special that needed to be done to use postfix instead, or was 
sendmail a hard requirement? I'm getting some pushback on using sendmail 
as listed in the Bro docs as the system admins prefer to use postfix.