dns: extend CURLOPT_RESOLVE syntax for adding non-permanent entries

Extend the syntax of CURLOPT_RESOLVE strings: allow using a '+' prefix
(similar to the existing '-' prefix for removing entries) to add
DNS cache entries that will time out just like entries that are added
by libcurl itself.

Append " (non-permanent)" to info log message in case a non-permanent
entry is added.

Adjust relevant comments to reflect the new behavior.

Adjust documentation.

Extend unit1607 to test the new functionality.

Closes #6294

1 files changed, 8 insertions, 5 deletions

diff --git a/lib/setopt.c b/lib/setopt.c
index a6cc562eb..cd7ec3fa8 100644
--- a/lib/setopt.c
+++ b/lib/setopt.c
@@ -1460,13 +1460,16 @@ CURLcode Curl_vsetopt(struct Curl_easy *data, CURLoption option, va_list param)
     break;
   case CURLOPT_RESOLVE:
     /*
-     * List of NAME:[address] names to populate the DNS cache with
-     * Prefix the NAME with dash (-) to _remove_ the name from the cache.
-     *
-     * Names added with this API will remain in the cache until explicitly
+     * List of HOST:PORT:[addresses] strings to populate the DNS cache with
+     * Entries added this way will remain in the cache until explicitly
      * removed or the handle is cleaned up.
      *
-     * This API can remove any name from the DNS cache, but only entries
+     * Prefix the HOST with plus sign (+) to have the entry expire just like
+     * automatically added entries.
+     *
+     * Prefix the HOST with dash (-) to _remove_ the entry from the cache.
+     *
+     * This API can remove any entry from the DNS cache, but only entries
      * that aren't actually in use right now will be pruned immediately.
      */
     data->set.resolve = va_arg(param, struct curl_slist *);