trace: omit first entry of the backtrace

The first entry in a backtrace tends to be the point in which we arrived at the
trace, which is a logtrace function. That only clutters output, because that is
a long named function that is basically adding no meaningful information

While we could pop the first element unconditionally, that would not be robust
against function inlining by the compiler. We can also add more trace functions
in the future, so I'm popping elements in a loop until we're out of trace stuff
in the beginning of the backtrace.

Backtrace traces are a lot easier to follow with this patch.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>