change "mox setaccountpassword" to use an account name as parameter, not email address

because with the name you would expect an account name.
and the email-resolving behaviour is surprising: with wildcard addresses you
can use any address, including a typo. you would change the password of the
address with the wildcard, without any warning. accounts are more precise and
less error-prone.

for issue #68 by x8x

docker-compose-imaptest.yml

@@ -10,7 +10,7 @@ services:
       - ./testdata/imaptest/imaptest.mbox:/mox/imaptest.mbox
     working_dir: /mox
     tty: true # For job control with set -m.
-    command: sh -c 'set -m; mox serve & sleep 1; echo testtest | mox setaccountpassword mjl@mox.example; fg'
+    command: sh -c 'set -m; mox serve & sleep 1; echo testtest | mox setaccountpassword mjl; fg'
     healthcheck:
       test: netstat -nlt | grep ':1143 '
       interval: 1s