add reverse ip checks during quickstart and in "check dns" admin page/subcommand

- and don't have a global variable "d" in the big checkDomain function in http/admin.go.
- and set loglevel from command-line flag again after loading the config file, for all subcommands except "serve".

import.go

@@ -19,7 +19,6 @@ import (
 	"github.com/mjl-/mox/message"
 	"github.com/mjl-/mox/metrics"
 	"github.com/mjl-/mox/mlog"
-	"github.com/mjl-/mox/mox-"
 	"github.com/mjl-/mox/store"
 )
 
@@ -86,7 +85,7 @@ func xcmdImport(mbox, train, markRead bool, args []string, c *cmd) {
 		c.Usage()
 	}
 
-	mox.MustLoadConfig()
+	mustLoadConfig()
 
 	account := args[0]
 	mailbox := args[1]