Fix building of mongoose under FreeBSD.

According to flockfile(3) under Linux you need only this:

    #define _XOPEN_SOURCE

* http://man7.org/linux/man-pages/man3/flockfile.3.html

This also fixes the build under FreeBSD.

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()