Rename httpd.conf "root strip" option to "request strip".

"root strip" was semantically incorrect and did cause some confusion
as it never stripped the root but the client's request path.
Discussed with many.  Heads up: this is a grammar change that also
affects acme-client(1) configurations (see current.html).
OK claudio@

src/etc/examples/httpd.conf

@@ -1,10 +1,10 @@
-# $OpenBSD: httpd.conf,v 1.19 2018/04/11 15:51:50 florian Exp $
+# $OpenBSD: httpd.conf,v 1.20 2018/06/13 15:08:24 reyk Exp $
 
 server "example.com" {
 	listen on * port 80
 	location "/.well-known/acme-challenge/*" {
 		root "/acme"
-		root strip 2
+		request strip 2
 	}
 	location * {
 		block return 302 "https://$HTTP_HOST$REQUEST_URI"
@@ -22,6 +22,6 @@ server "example.com" {
 	}
 	location "/.well-known/acme-challenge/*" {
 		root "/acme"
-		root strip 2
+		request strip 2
 	}
 }