allow unsetting a log level through subcommand and add admin page for settng log level

main.go

@@ -745,6 +745,8 @@ By default, a single log level applies to all logging in mox. But for each
 smtpserver, smtpclient, queue, imapserver, spf, dkim, dmarc, junk, message,
 etc.
 
+Specify a pkg and an empty level to clear the configured level for a package.
+
 Valid labels: error, info, debug, trace, traceauth, tracedata.
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