variable which is de-facto standard for MUAs.
Teach bomail to generate an in-reply-to header so threading MUAs and
mail->news gateways won't lose context.
While i was at it, removed two gratuitous standard violations for
functions starting with an underscore.
plain 0 should be used. This happens to work because we #define
NULL to 0, but is stylistically wrong and can cause problems
for people trying to port bits of code to other environments.
PR: 2752
Submitted by: Arne Henrik Juul <arnej@imf.unit.no>
