Add filtering for curl debug output.

Add CURLTR debug channel for libcurl API calls,
and use CURLIO only for libcurl debug output.
Note: need to set gDebugCurlTerse to true for
filtering to take effect, then pass 'debug_on'
to the LLHttpClient methods that require debugging.

indra/cwdebug/debug.h

@@ -207,6 +207,7 @@ extern CWD_API channel_ct statemachine;
 extern CWD_API channel_ct caps;
 extern CWD_API channel_ct curl;
 extern CWD_API channel_ct curlio;
+extern CWD_API channel_ct curltr;
 
 #endif