for WebRedirect, don't "match" when the destination URL has the same scheme,host,path, for doing http -> https redirects without loops

you can already get most http to https redirects through DontRedirectPlainHTTP
in WebHandler, but that needs handlers for all paths.

now you can just set up a redirect for a domain and all its path to baseurl
https://domain (leaving other webdirect fields empty). when the request comes
in with plain http, the redirect to https is done. that next request will also
evaluate the same redirect rule. but it will not cause a match because it would
redirect to the same scheme,host,path. so next webhandlers get a chance to
serve.

also clarify in webhandlers docs that also account & admin built-in handlers
run first.

related to issue #16

config/config.go

@@ -73,7 +73,7 @@ type Dynamic struct {
 	Domains            map[string]Domain  `sconf-doc:"Domains for which email is accepted. For internationalized domains, use their IDNA names in UTF-8."`
 	Accounts           map[string]Account `sconf-doc:"Accounts to which email can be delivered. An account can accept email for multiple domains, for multiple localparts, and deliver to multiple mailboxes."`
 	WebDomainRedirects map[string]string  `sconf:"optional" sconf-doc:"Redirect all requests from domain (key) to domain (value). Always redirects to HTTPS. For plain HTTP redirects, use a WebHandler with a WebRedirect."`
-	WebHandlers        []WebHandler       `sconf:"optional" sconf-doc:"Handle webserver requests by serving static files, redirecting or reverse-proxying HTTP(s). The first matching WebHandler will handle the request. Built-in handlers for autoconfig and mta-sts always run first. If no handler matches, the response status code is file not found (404). If functionality you need is missng, simply forward the requests to an application that can provide the needed functionality."`
+	WebHandlers        []WebHandler       `sconf:"optional" sconf-doc:"Handle webserver requests by serving static files, redirecting or reverse-proxying HTTP(s). The first matching WebHandler will handle the request. Built-in handlers, e.g. for account, admin, autoconfig and mta-sts always run first. If no handler matches, the response status code is file not found (404). If functionality you need is missng, simply forward the requests to an application that can provide the needed functionality."`
 
 	WebDNSDomainRedirects map[dns.Domain]dns.Domain `sconf:"-"`
 }
@@ -366,7 +366,7 @@ type WebStatic struct {
 }
 
 type WebRedirect struct {
-	BaseURL        string `sconf:"optional" sconf-doc:"Base URL to redirect to. The path must be empty and will be replaced, either by the request URL path, or by OrigPathRegexp/ReplacePath. Scheme, host, port and fragment stay intact, and query strings are combined. If empty, the response redirects to a different path through OrigPathRegexp and ReplacePath, which must then be set. Use a URL without scheme to redirect without changing the protocol, e.g. //newdomain/."`
+	BaseURL        string `sconf:"optional" sconf-doc:"Base URL to redirect to. The path must be empty and will be replaced, either by the request URL path, or by OrigPathRegexp/ReplacePath. Scheme, host, port and fragment stay intact, and query strings are combined. If empty, the response redirects to a different path through OrigPathRegexp and ReplacePath, which must then be set. Use a URL without scheme to redirect without changing the protocol, e.g. //newdomain/. If a redirect would send a request to a URL with the same scheme, host and path, the WebRedirect does not match so a next WebHandler can be tried. This can be used to redirect all plain http traffic to https."`
 	OrigPathRegexp string `sconf:"optional" sconf-doc:"Regular expression for matching path. If set and path does not match, a 404 is returned. The HTTP path used for matching always starts with a slash."`
 	ReplacePath    string `sconf:"optional" sconf-doc:"Replacement path for destination URL based on OrigPathRegexp. Implemented with Go's Regexp.ReplaceAllString: $1 is replaced with the text of the first submatch, etc. If both OrigPathRegexp and ReplacePath are empty, BaseURL must be set and all paths are redirected unaltered."`
 	StatusCode     int    `sconf:"optional" sconf-doc:"Status code to use in redirect, e.g. 307. By default, a permanent redirect (308) is returned."`