check and log errors more often in deferred cleanup calls, and log remote-induced errors at lower priority

We normally check errors for all operations. But for some cleanup calls, eg
"defer file.Close()", we didn't. Now we also check and log most of those.
Partially because those errors can point to some mishandling or unexpected code
paths (eg file unexpected already closed). And in part to make it easier to use
"errcheck" to find the real missing error checks, there is too much noise now.

The log.Check function can now be used unconditionally for checking and logging
about errors. It adjusts the log level if the error is caused by a network
connection being closed, or a context is canceled or its deadline reached, or a
socket deadline is reached.

main.go

@@ -1106,7 +1106,9 @@ func ctlcmdConfigTlspubkeyGet(ctl *ctl, fingerprint string) {
 	fmt.Printf("type: %s\nname: %s\naccount: %s\naddress: %s\nno imap preauth: %s\n", typ, name, account, address, noimappreauth)
 	if block != nil {
 		fmt.Printf("certificate:\n\n")
-		pem.Encode(os.Stdout, block)
+		if err := pem.Encode(os.Stdout, block); err != nil {
+			log.Fatalf("pem encode: %v", err)
+		}
 	}
 }
 
@@ -1918,7 +1920,8 @@ connection.
 	go func() {
 		_, err := io.Copy(os.Stdout, conn)
 		xcheckf(err, "copy from connection to stdout")
-		conn.Close()
+		err = conn.Close()
+		c.log.Check(err, "closing connection")
 	}()
 	_, err = io.Copy(conn, os.Stdin)
 	xcheckf(err, "copy from stdin to connection")
@@ -2079,7 +2082,9 @@ sharing most of its code.
 		sc, err := smtpclient.New(ctxbg, c.log.Logger, conn, tlsMode, tlsPKIX, ehloDomain, tlsHostnames[0], opts)
 		if err != nil {
 			log.Printf("setting up smtp session: %v, skipping", err)
-			conn.Close()
+			if xerr := conn.Close(); xerr != nil {
+				log.Printf("closing connection: %v", xerr)
+			}
 			continue
 		}
 
@@ -2093,7 +2098,9 @@ sharing most of its code.
 		go func() {
 			_, err := io.Copy(os.Stdout, smtpConn)
 			xcheckf(err, "copy from connection to stdout")
-			smtpConn.Close()
+			if err := smtpConn.Close(); err != nil {
+				log.Printf("closing smtp connection: %v", err)
+			}
 		}()
 		_, err = io.Copy(smtpConn, os.Stdin)
 		xcheckf(err, "copy from stdin to connection")
@@ -2524,7 +2531,11 @@ headers prepended.
 
 	msgf, err := os.Open(args[0])
 	xcheckf(err, "open message")
-	defer msgf.Close()
+	defer func() {
+		if err := msgf.Close(); err != nil {
+			log.Printf("closing message file: %v", err)
+		}
+	}()
 
 	p, err := message.Parse(c.log.Logger, true, msgf)
 	xcheckf(err, "parsing message")
@@ -3019,7 +3030,7 @@ automation.
 
 	domain := xparseDomain(args[0], "domain")
 
-	registration, err := rdap.LookupLastDomainRegistration(context.Background(), domain)
+	registration, err := rdap.LookupLastDomainRegistration(context.Background(), c.log, domain)
 	xcheckf(err, "looking up domain in rdap")
 
 	age := time.Since(registration)
@@ -3278,7 +3289,11 @@ func cmdWebapi(c *cmd) {
 
 	resp, err := http.PostForm(args[1]+args[0], url.Values{"request": []string{string(request)}})
 	xcheckf(err, "http post")
-	defer resp.Body.Close()
+	defer func() {
+		if err := resp.Body.Close(); err != nil {
+			log.Printf("closing http response body: %v", err)
+		}
+	}()
 	if resp.StatusCode == http.StatusBadRequest {
 		buf, err := io.ReadAll(&moxio.LimitReader{R: resp.Body, Limit: 10 * 1024})
 		xcheckf(err, "reading response for 400 bad request error")
@@ -3681,7 +3696,11 @@ func cmdMessageParse(c *cmd) {
 
 	f, err := os.Open(args[0])
 	xcheckf(err, "open")
-	defer f.Close()
+	defer func() {
+		if err := f.Close(); err != nil {
+			log.Printf("closing message file: %v", err)
+		}
+	}()
 
 	part, err := message.Parse(c.log.Logger, false, f)
 	xcheckf(err, "parsing message")
@@ -3892,7 +3911,11 @@ Opens database files directly, not going through a running mox instance.
 						buf := headerbuf[:int(size)]
 						err := func() error {
 							mr := a.MessageReader(m)
-							defer mr.Close()
+							defer func() {
+								if err := mr.Close(); err != nil {
+									log.Printf("closing message reader: %v", err)
+								}
+							}()
 
 							// ReadAt returns whole buffer or error. Single read should be fast.
 							n, err := mr.ReadAt(buf, partialPart.HeaderOffset)