add basic webserver that can do most of what i need

- serve static files, serving index.html or optionally listings for directories
- redirects
- reverse-proxy, forwarding requests to a backend

these are configurable through the config file. a domain and path regexp have to
be configured. path prefixes can be stripped.  configured domains are added to
the autotls allowlist, so acme automatically fetches certificates for them.

all webserver requests now have (access) logging, metrics, rate limiting.
on http errors, the error message prints an encrypted cid for relating with log files.

this also adds a new mechanism for example config files.

main.go

@@ -105,6 +105,7 @@ var commands = []struct {
 	{"config domain rm", cmdConfigDomainRemove},
 	{"config describe-sendmail", cmdConfigDescribeSendmail},
 	{"config printservice", cmdConfigPrintservice},
+	{"examples", cmdExamples},
 
 	{"checkupdate", cmdCheckupdate},
 	{"cid", cmdCid},
@@ -758,6 +759,109 @@ func cmdConfigDNSCheck(c *cmd) {
 	printResult("Autodiscover", result.Autodiscover.Result)
 }
 
+var examples = []struct {
+	Name string
+	Get  func() string
+}{
+	{
+		"webhandlers",
+		func() string {
+			const webhandlers = `# Snippet of domains.conf to configure WebHandlers.
+WebHandlers:
+	-
+		# The name of the handler, used in logging and metrics.
+		LogName: staticmjl
+		# With ACME configured, each configured domain will automatically get a TLS
+		# certificate on first request.
+		Domain: mox.example
+		PathRegexp: ^/who/mjl/
+		WebStatic:
+			StripPrefix: /who/mjl
+			# Requested path /who/mjl/inferno/ resolves to local web/mjl/inferno.
+			# If a directory contains an index.html, it is served when a directory is requested.
+			Root: web/mjl
+			# With ListFiles true, if a directory does not contain an index.html, the contents are listed.
+			ListFiles: true
+			ResponseHeaders:
+				X-Mox: hi
+	-
+		LogName: redir
+		Domain: mox.example
+		PathRegexp: ^/redir/a/b/c
+		# Don't redirect from plain HTTP to HTTPS.
+		DontRedirectPlainHTTP: true
+		WebRedirect:
+			# Just change the domain and add query string set fragment. No change to scheme.
+			# Path will start with /redir/a/b/c (and whathever came after) because no
+			# OrigPathRegexp+ReplacePath is set.
+			BaseURL: //moxest.example?q=1#frag
+			# Default redirection is 308 - Permanent Redirect.
+			StatusCode: 307
+	-
+		LogName: oldnew
+		Domain: mox.example
+		PathRegexp: ^/old/
+		WebRedirect:
+			# Replace path, leaving rest of URL intact.
+			OrigPathRegexp: ^/old/(.*)
+			ReplacePath: /new/$1
+	-
+		LogName: app
+		Domain: mox.example
+		PathRegexp: ^/app/
+		WebForward:
+			# Strip the path matched by PathRegexp before forwarding the request. So original
+			# request /app/api become just /api.
+			StripPath: true
+			# URL of backend, where requests are forwarded to. The path in the URL is kept,
+			# so for incoming request URL /app/api, the outgoing request URL has path /app-v2/api.
+			# Requests are made with Go's net/http DefaultTransporter, including using
+			# HTTP_PROXY and HTTPS_PROXY environment variables.
+			URL: http://127.0.0.1:8900/app-v2/
+			# Add headers to response.
+			ResponseHeaders:
+				X-Frame-Options: deny
+				X-Content-Type-Options: nosniff
+`
+			// Parse just so we know we have the syntax right.
+			// todo: ideally we would have a complete config file and parse it fully.
+			var conf struct {
+				WebHandlers []config.WebHandler
+			}
+			err := sconf.Parse(strings.NewReader(webhandlers), &conf)
+			xcheckf(err, "parsing webhandlers example")
+			return webhandlers
+		},
+	},
+}
+
+func cmdExamples(c *cmd) {
+	c.params = "[name]"
+	c.help = `List available examples, or print a specific example.`
+
+	args := c.Parse()
+	if len(args) > 1 {
+		c.Usage()
+	}
+
+	var match func() string
+	for _, ex := range examples {
+		if len(args) == 0 {
+			fmt.Println(ex.Name)
+		} else if args[0] == ex.Name {
+			match = ex.Get
+		}
+	}
+	if len(args) == 0 {
+		return
+	}
+	if match == nil {
+		log.Fatalln("not found")
+	}
+	fmt.Print(match())
+	return
+}
+
 func cmdLoglevels(c *cmd) {
 	c.params = "[level [pkg]]"
 	c.help = `Print the log levels, or set a new default log level, or a level for the given package.