Improve tracef/tracelog to use the stack for small strings

Support two common cases, one being that the resulting message is
small enough to fit into a on-stack buffer.
The seconds being the common 'printf("%s", "Message")' scheme.

Unfortunately, iterating a va_list is destructive,
so it has to be copied before calling vprintf.

The implementation was moved to a separate file,
used by both tracef.c and tracelog.c.

Signed-off-by: Norbert Lange <nolange79@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>