httpd: raise the default web session timeout to one hour

200 seconds was too short in practice: the UI stops polling while its
tab is hidden, so looking at another tab for a few minutes ended the
session. Make it 3600 seconds; "session <seconds>" still overrides it.

httpd/httpd.c

@@ -12,7 +12,7 @@
 // #define DEBUG
 #include "debug.h"
 
-#define SESSION_TIMEOUT_DEFAULT 200
+#define SESSION_TIMEOUT_DEFAULT 3600
 
 /* Seconds of inactivity after which a session cookie stops validating.
  * Configurable with the "session" command, see cmd_parser.c. */