Don't allow mailboxes named "." or ".." and normalize names during imports too

It only serves to confuse. When exporting such mailboxes in zip files or tar
files, extracting will cause trouble.

import.go

@@ -16,6 +16,8 @@ import (
 	"strings"
 	"time"
 
+	"golang.org/x/text/unicode/norm"
+
 	"github.com/mjl-/mox/config"
 	"github.com/mjl-/mox/message"
 	"github.com/mjl-/mox/metrics"
@@ -189,6 +191,11 @@ func importctl(ctx context.Context, ctl *ctl, mbox bool) {
 	var mdnewf, mdcurf *os.File
 	var msgreader store.MsgSource
 
+	// Ensure normalized form.
+	mailbox = norm.NFC.String(mailbox)
+	mailbox, _, err = store.CheckMailboxName(mailbox, true)
+	ctl.xcheck(err, "checking mailbox name")
+
 	// Open account, creating a database file if it doesn't exist yet. It must be known
 	// in the configuration file.
 	a, err := store.OpenAccount(ctl.log, account, false)