multiplex: enable by default

Starting 7.62.0, multiplexing is enabled by default in multi handles.

docs/libcurl/opts/CURLMOPT_PIPELINING.3

@@ -63,7 +63,9 @@ This bit is deprecated and has no effect since version 7.62.0.
 If this bit is set, libcurl will try to multiplex the new transfer over an
 existing connection if possible. This requires HTTP/2.
 .SH DEFAULT
-0 (both pipeline and multiplexing are off)
+Since 7.62.0, \fBCURLPIPE_MULTIPLEX\bP is enabled by default.
+
+Before that, default was \fBCURLPIPE_NOTHING\fP.
 .SH PROTOCOLS
 HTTP(S)
 .SH EXAMPLE

lib/multi.c

@@ -347,6 +347,7 @@ struct Curl_multi *Curl_multi_handle(int hashsize, /* socket hash */
   Curl_llist_init(&multi->pending, multi_freeamsg);
 
   multi->max_pipeline_length = 5;
+  multi->pipelining = CURLPIPE_MULTIPLEX;
 
   /* -1 means it not set by user, use the default value */
   multi->maxconnects = -1;