Introduce vtracef

Introduce vtracef

vtracef accepts a va_list argument to simplify
tracing functions which use a va_list

Here's an example from wpa_supplicant that I wanted to
trace:

void wpa_debug(int level, const char* fmt, ...)
{
	va_list ap;
	va_start(ap, fmt);

	...
	// The call I want to easily trace with vtracef
	vprintf(fmt, ap);

	...
	va_end(ap);
}

wpa_debug is used a fair amount and it would be annoying to
replace all the wpa_debug calls with tracef.

With vtracef, it simplifies the find and replace effort by
only changing it at one place.

Signed-off-by: Maxime Roussin-Belanger <maxime.roussinbelanger@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>