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
The alt-svc cache survives a call to curl_easy_reset fine, but the file
name to use for saving the cache was cleared. Now the alt-svc cache has
a copy of the file name to survive handle resets.
Added test 1908 to verify.
Reported-by: Craig Andrews
Fixes#4898Closes#4902
