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

localserve.go

@@ -218,7 +218,7 @@ during those commands instead of during "data".
 			}
 			cid := mox.Cid()
 			ctx := context.WithValue(mox.Context, mlog.CidKey, cid)
-			go servectl(ctx, log.WithCid(cid), conn, func() { shutdown(log) })
+			go servectl(ctx, cid, log.WithCid(cid), conn, func() { shutdown(log) })
 		}
 	}()