add aliases/lists: when sending to an alias, the message gets delivered to all members

the members must currently all be addresses of local accounts.

a message sent to an alias is accepted if at least one of the members accepts
it. if no members accepts it (e.g. due to bad reputation of sender), the
message is rejected.

if a message is submitted to both an alias addresses and to recipients that are
members of the alias in an smtp transaction, the message will be delivered to
such members only once.  the same applies if the address in the message
from-header is the address of a member: that member won't receive the message
(they sent it). this prevents duplicate messages.

aliases have three configuration options:
- PostPublic: whether anyone can send through the alias, or only members.
  members-only lists can be useful inside organizations for internal
  communication. public lists can be useful for support addresses.
- ListMembers: whether members can see the addresses of other members. this can
  be seen in the account web interface. in the future, we could export this in
  other ways, so clients can expand the list.
- AllowMsgFrom: whether messages can be sent through the alias with the alias
  address used in the message from-header. the webmail knows it can use that
  address, and will use it as from-address when replying to a message sent to
  that address.

ideas for the future:
- allow external addresses as members. still with some restrictions, such as
  requiring a valid dkim-signature so delivery has a chance to succeed. will
  also need configuration of an admin that can receive any bounces.
- allow specifying specific members who can sent through the list (instead of
  all members).

for github issue #57 by hmfaysal.
also relevant for #99 by naturalethic.
thanks to damir & marin from sartura for discussing requirements/features.

config/doc.go

@@ -913,6 +913,31 @@ See https://pkg.go.dev/github.com/mjl-/sconf for details.
 					MinimumAttempts: 0
 					Transport:
 
+			# Aliases that cause messages to be delivered to one or more locally configured
+			# addresses. Keys are localparts (encoded, as they appear in email addresses).
+			# (optional)
+			Aliases:
+				x:
+
+					# Expanded addresses to deliver to. These must currently be of addresses of local
+					# accounts. To prevent duplicate messages, a member address that is also an
+					# explicit recipient in the SMTP transaction will only have the message delivered
+					# once. If the address in the message From header is a member, that member also
+					# won't receive the message.
+					Addresses:
+						-
+
+					# If true, anyone can send messages to the list. Otherwise only members, based on
+					# message From address, which is assumed to be DMARC-like-verified. (optional)
+					PostPublic: false
+
+					# If true, members can see addresses of members. (optional)
+					ListMembers: false
+
+					# If true, members are allowed to send messages with this alias address in the
+					# message From header. (optional)
+					AllowMsgFrom: false
+
 	# Accounts represent mox users, each with a password and email address(es) to
 	# which email can be delivered (possibly at different domains). Each account has
 	# its own on-disk directory holding its messages and index database. An account