Allow debug tracing to be disabled in debug-mode build by defining DEBUG_TRACE at compile-time.

Added printf format checking macros. Changed mg_printf() to allocate the buffer on heap if it does not fit the local one.

backport of the relevant fix as per issue 370#c5: correct get_request under all circumstances

This reverts commit 15a0d819.
There is no point in MG_SHUTDOWN. Thread that calls mg_stop() can
do the same thing.

Changing API: callback doesnt get mg_request_info pointer anymore, but it is possible to get it using mg_get_request_info()

Changing API: callback doesnt get mg_request_info pointer anymore, but it is possible to get it using mg_get_request_info()

Added .json => text/json mime type. In handle_cgi_request, closing write stream straight after data is sent to the CGI process.