Allow user to display status code errors

This reverts commit 27fe9c8f.

Allows user code to display its own error status responses or just log
the errors. Is called even in cases where begin_request is not called.

Made behavior of mg_get_cookie(..) on errors the same as mg_get_var(..). Separated the two cases a.) problems with destination buffer and b.) cookie header or cookie parameter not existing.

Made the names of the function arguments of the declaration and the definition of mg_get_var(..) the same.

Changed the error codes to have a consistent way of reporting errors. -2 is returned if there is a problem with the size of the destination buffer. -1 is returned if the variable is not found.

Updated the documentation of mg_get_var(..) in mongoose.h and removed the redundant comments from mongoose.c.

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

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