altsvc: clone setting in curl_easy_duphandle

The cache content is not duplicated, like other caches, but the setting
and specified file name are.

Test 1908 is extended to verify this somewhat. Since the duplicated
handle gets the same file name, the test unfortunately overwrites the
same file twice (with different contents) which makes it hard to check
automatically.

Closes #5923

tests/data/test1908

@@ -8,7 +8,7 @@ CURLINFO_EFFECTIVE_URL
 # Server-side
 <reply>
 <data nocheck="yes">
-HTTP/1.1 200 OK
+HTTP/1.1 200 OK swsbounce
 Date: Thu, 09 Nov 2010 14:49:00 GMT
 Server: test-server/fake
 Content-Type: text/html
@@ -17,6 +17,15 @@ Content-Length: 0
 alt-svc: h2="3dbbdetxoyw4nsp6c3cc456oj2ays6s43ezxzsfxxri3h5xqd.example:443"; ma=315360000; persist=1
 
 </data>
+
+# This is the second response
+<data1 nocheck="yes">
+HTTP/1.1 200 OK
+Connection: close
+Content-Length: 0
+alt-svc: h2="second.example:443"; ma=315360000;
+
+</data1>
 </reply>
 
 # Client-side
@@ -56,6 +65,10 @@ GET /1908 HTTP/1.1
 Host: %HOSTIP:%HTTPPORT
 Accept: */*
 
+GET /1908 HTTP/1.1
+Host: %HOSTIP:%HTTPPORT
+Accept: */*
+
 </protocol>
 <stripfile>
 # strip out the (dynamic) expire date from the file so that the rest