add cli command "mox admin imapserve $preauthaddress"

for admins to open an imap connection preauthenticated for an account (by address), also when
it is disabled for logins.

useful for migrations. the admin typically doesn't know the password of the
account, so couldn't get an imap session (for synchronizing) before.

tested with "mox localserve" and running:

	mutt -e 'set tunnel="MOXCONF=/home/mjl/.config/mox-localserve/mox.conf ./mox admin imapserve mox@localhost"'

may also work with interimap, but untested.

i initially assumed imap would be done fully on file descriptor 0, but mutt
expects imap output on fd 1. that's the default now. flag -fd0 is for others
that expect it on fd0.

for issue #175, suggested by DanielG

doc.go

@@ -89,6 +89,7 @@ any parameters. Followed by the help and usage information for each command.
 	mox config printservice >mox.service
 	mox config ensureacmehostprivatekeys
 	mox config example [name]
+	mox admin imapserve preauth-address
 	mox checkupdate
 	mox cid cid
 	mox clientconfig domain
@@ -1204,6 +1205,18 @@ List available config examples, or print a specific example.
 
 	usage: mox config example [name]
 
+# mox admin imapserve
+
+Initiate a preauthenticated IMAP connection on file descriptor 0.
+
+For use with tools that can do IMAP over tunneled connections, e.g. with SSH
+during migrations. TLS is not possible on the connection, and authentication
+does not require TLS.
+
+	usage: mox admin imapserve preauth-address
+	  -fd0
+	    	write IMAP to file descriptor 0 instead of stdout
+
 # mox checkupdate
 
 Check if a newer version of mox is available.