gptel-curl: Replace Curl timeout with speed-time

* gptel-curl.el (gptel-curl--common-args): Following the
discussion in #143, Use "-y300 -Y1" as Curl arguments instead of
specifying the timeout.  Now the connection stays open unless less
than 1 byte of information is exchanged over 300 seconds.

gptel-curl.el

@@ -37,9 +37,9 @@
 (defconst gptel-curl--common-args
   (if (memq system-type '(windows-nt ms-dos))
       '("--disable" "--location" "--silent" "-XPOST"
-        "-m300" "-D-")
+        "-y300" "-Y1" "-D-")
     '("--disable" "--location" "--silent" "--compressed"
-      "-XPOST" "-m300" "-D-"))
+      "-XPOST" "-y300" "-Y1" "-D-"))
   "Arguments always passed to Curl for gptel queries.")
 
 (defvar gptel-curl--process-alist nil