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.

doc.go

@@ -68,6 +68,13 @@ any parameters. Followed by the help and usage information for each command.
 	mox config address rm address
 	mox config domain add domain account [localpart]
 	mox config domain rm domain
+	mox config alias list domain
+	mox config alias print alias
+	mox config alias add alias@domain rcpt1@domain ...
+	mox config alias update alias@domain [-postpublic false|true -listmembers false|true -allowmsgfrom false|true]
+	mox config alias rm alias@domain
+	mox config alias addaddr alias@domain rcpt1@domain ...
+	mox config alias rmaddr alias@domain rcpt1@domain ...
 	mox config describe-sendmail >/etc/moxsubmit.conf
 	mox config printservice >mox.service
 	mox config ensureacmehostprivatekeys
@@ -968,6 +975,54 @@ rejected.
 
 	usage: mox config domain rm domain
 
+# mox config alias list
+
+List aliases for domain.
+
+	usage: mox config alias list domain
+
+# mox config alias print
+
+Print settings and members of alias.
+
+	usage: mox config alias print alias
+
+# mox config alias add
+
+Add new alias with one or more addresses.
+
+	usage: mox config alias add alias@domain rcpt1@domain ...
+
+# mox config alias update
+
+Update alias configuration.
+
+	usage: mox config alias update alias@domain [-postpublic false|true -listmembers false|true -allowmsgfrom false|true]
+	  -allowmsgfrom string
+	    	whether alias address can be used in message from header
+	  -listmembers string
+	    	whether list members can list members
+	  -postpublic string
+	    	whether anyone or only list members can post
+
+# mox config alias rm
+
+Remove alias.
+
+	usage: mox config alias rm alias@domain
+
+# mox config alias addaddr
+
+Add addresses to alias.
+
+	usage: mox config alias addaddr alias@domain rcpt1@domain ...
+
+# mox config alias rmaddr
+
+Remove addresses from alias.
+
+	usage: mox config alias rmaddr alias@domain rcpt1@domain ...
+
 # mox config describe-sendmail
 
 Describe configuration for mox when invoked as sendmail.