libssh2_trace_sethandler installs a trace output handler for your application.
By default, when tracing has been switched on via a call to libssh2_trace(),
all output is written to stderr. By calling this method and passing a
function pointer that matches the libssh2_trace_handler_func prototype,
libssh2 will call back as it generates trace output. This can be used to
capture the trace output and put it into a log file or diagnostic window.
This function has no effect unless libssh2 was built to support this option,
and a typical "release build" might not.
context can be used to pass arbitrary user defined data back into the callback when invoked.