manual.md: avoid external calls in Procmail recipe; bounce on error

[tripleee]

This was brought to my attention via a question on Server Fault: https://serverfault.com/questions/1163955/how-do-i-solve-procmail-not-writing-to-home-maildir/1163956

This proposed change introduces the following changes:

• If lookup-domain.pl fails, we generate a bounce message, instead of silently discarding email to /dev/null and pretending everything is fine.
• This avoids the unnecessary external calls to test to perform string comparisons, which Procmail out of the box does very well by itself.

This only changes the manual installation instructions. I went looking for a file which would contain something similar for non-manual installations, but could not find it. Arguably if there is such a thing, it should be kept in sync.

[iliajie]

Hello,

Thank you for your suggestion. However, it's important to note that this is how we currently manage it on the Virtualmin side. If you'd like to make a suggestion, please open a ticket at virtualmin/virtualmin-gpl instead.