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!

webadmin/api.json

@@ -6528,6 +6528,14 @@
 						"WebForward"
 					]
 				},
+				{
+					"Name": "WebInternal",
+					"Docs": "",
+					"Typewords": [
+						"nullable",
+						"WebInternal"
+					]
+				},
 				{
 					"Name": "Name",
 					"Docs": "Either LogName, or numeric index if LogName was empty. Used instead of LogName in logging/metrics.",
@@ -6648,6 +6656,26 @@
 				}
 			]
 		},
+		{
+			"Name": "WebInternal",
+			"Docs": "",
+			"Fields": [
+				{
+					"Name": "BasePath",
+					"Docs": "",
+					"Typewords": [
+						"string"
+					]
+				},
+				{
+					"Name": "Service",
+					"Docs": "",
+					"Typewords": [
+						"string"
+					]
+				}
+			]
+		},
 		{
 			"Name": "Transport",
 			"Docs": "Transport is a method to delivery a message. At most one of the fields can\nbe non-nil. The non-nil field represents the type of transport. For a\ntransport with all fields nil, regular email delivery is done.",