improve http request handling for internal services and multiple domains

per listener, you could enable the admin/account/webmail/webapi handlers. but
that would serve those services on their configured paths (/admin/, /,
/webmail/, /webapi/) on all domains mox would be webserving, including any
non-mail domains. so your www.example/admin/ would be serving the admin web
interface, with no way to disabled that.

with this change, the admin interface is only served on requests to (based on
Host header):
- ip addresses
- the listener host name (explicitly configured in the listener, with fallback
  to global hostname)
- "localhost" (for ssh tunnel/forwarding scenario's)

the account/webmail/webapi interfaces are served on the same domains as the
admin interface, and additionally:
- the client settings domains, as optionally configured in each Domain in
  domains.conf. typically "mail.<yourdomain>".

this means the internal services are no longer served on other domains
configured in the webserver, e.g. www.example.org/admin/ will not be handled
specially.

the order of evaluation of routes/services is also changed:
before this change, the internal handlers would always be evaluated first.
with this change, only the system handlers for
MTA-STS/autoconfig/ACME-validation will be evaluated first. then the webserver
handlers. and finally the internal services (admin/account/webmail/webapi).
this allows an admin to configure overrides for some of the domains (per
hostname-matching rules explained above) that would normally serve these
services.

webserver handlers can now be configured that pass the request to an internal
service: in addition to the existing static/redirect/forward config options,
there is now an "internal" config option, naming the service
(admin/account/webmail/webapi) for handling the request. this allows enabling
the internal services on custom domains.

for issue #160 by TragicLifeHu, thanks for reporting!

testdata/web/domains.conf

@@ -1,6 +1,12 @@
 Domains:
 	mox.example:
 		LocalpartCaseSensitive: false
+		ClientSettingsDomain: mail.mox.example
+		MTASTS:
+			PolicyID: 1
+			Mode: enforce
+			MaxAge: 24h
+	other.example: nil
 Accounts:
 	mjl:
 		Domain: mox.example
@@ -21,3 +27,10 @@ WebHandlers:
 			ListFiles: true
 			ResponseHeaders:
 				X-Test: mox
+	-
+		LogName: adminoverride
+		Domain: mox.example
+		PathRegexp: ^/admin/
+		DontRedirectPlainHTTP: true
+		WebRedirect:
+			BaseURL: http://redirect.example

testdata/web/mox.conf

@@ -8,6 +8,14 @@ Listeners:
 			- 0.0.0.0
 		WebserverHTTP:
 			Enabled: true
+		AdminHTTP:
+			Enabled: true
+		WebmailHTTP:
+			Enabled: true
+		MTASTSHTTPS:
+			Enabled: true
+			Port: 80
+			NonTLS: true
 Postmaster:
 	Account: mjl
 	Mailbox: postmaster

testdata/webserver/domains.conf

@@ -69,6 +69,38 @@ WebHandlers:
 			BaseURL: //other.mox.example?q=1#fragment
 			OrigPathRegexp: ^/baseurlpath/old/(.*)$
 			ReplacePath: /baseurlpath/new/$1
+	-
+		LogName: xadmin
+		Domain: mox.example
+		PathRegexp: ^/xadmin/
+		DontRedirectPlainHTTP: true
+		WebInternal:
+			BasePath: /xadmin/
+			Service: admin
+	-
+		LogName: xaccount
+		Domain: mox.example
+		PathRegexp: ^/xaccount/
+		DontRedirectPlainHTTP: true
+		WebInternal:
+			BasePath: /xaccount/
+			Service: account
+	-
+		LogName: xwebmail
+		Domain: mox.example
+		PathRegexp: ^/xwebmail/
+		DontRedirectPlainHTTP: true
+		WebInternal:
+			BasePath: /xwebmail/
+			Service: webmail
+	-
+		LogName: xwebapi
+		Domain: mox.example
+		PathRegexp: ^/xwebapi/
+		DontRedirectPlainHTTP: true
+		WebInternal:
+			BasePath: /xwebapi/
+			Service: webapi
 	# test code depends on these last two webhandlers being here.
 	-
 		LogName: strippath

testdata/webserver/mox.conf

@@ -8,6 +8,8 @@ Listeners:
 			- 0.0.0.0
 		WebserverHTTP:
 			Enabled: true
+		AdminHTTP:
+			Enabled: true
 Postmaster:
 	Account: mjl
 	Mailbox: postmaster