smtp-dummy: Prefer return rather than exit() in main.

smtp-dummy: Prefer return rather than exit() in main.

The main() function should be written as just another function with a
return value. This allows for more reliable code reuse. Imagine that
main() grows too large and needs to be factored into multiple
functions. At that point, exit() is probably the wrong thing, yet can
also be hard to notice as it's in less-frequently-tested exceptional
cases.