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/doc.go

@@ -570,10 +570,10 @@ describe-static" and "mox config describe-domains":
 
 	# 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. (optional)
+	# 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. (optional)
 	WebHandlers:
 		-
 
@@ -637,7 +637,10 @@ describe-static" and "mox config describe-domains":
 				# 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/. (optional)
+				# 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.
+				# (optional)
 				BaseURL:
 
 				# Regular expression for matching path. If set and path does not match, a 404 is
@@ -689,6 +692,20 @@ examples with "mox example", and print a specific example with "mox example
 
 	# Each request is matched against these handlers until one matches and serves it.
 	WebHandlers:
+		-
+			# Redirect all plain http requests to https, leaving path, query strings, etc
+			# intact. When the request is already to https, the destination URL would have the
+			# same scheme, host and path, causing this redirect handler to not match the
+			# request (and not cause a redirect loop) and the webserver to serve the request
+			# with a later handler.
+			LogName: redirhttps
+			Domain: www.mox.example
+			PathRegexp: ^/
+			# Could leave DontRedirectPlainHTTP at false if it wasn't for this being an
+			# example for doing this redirect.
+			DontRedirectPlainHTTP: true
+			WebRedirect:
+				BaseURL: https://www.mox.example
 		-
 			# The name of the handler, used in logging and metrics.
 			LogName: staticmjl