Add support for user+extension@domain addresses

[rjl]

Maia needs to become aware of the user+extension@domain address format, and should properly auto-link such addresses to the account that owns the user@domain address. While this can currently be done manually, there's no reason that Maia shouldn't be able to sense this on its own and automatically consolidate such accounts.

[jleaver]

Quite frankly, I'd consider not linking them. They aren't a separate email address, nor are they an alias. I'm of the opinion that +extension should be ignored for all intents and purposes, since they basically equate to a delivery instruction for the downstream MTA. The tricky part would be to ingore extensions while still managing to preserve them downstream.

[Barryf]

I would also like to see this feature implemented. There is some mention of $include_bare_user in the amavis code which looks like it is intended for this purpose, but I can't see where this is enabled.

[dmorton]

I'm not sure I like this either. Fixing your rewriting to do what you want before it gets to maia seems like the better choice