trace-cmd: allow python tools to supress warning() and pr_stat()

trace-cmd has a lot of built in warning() and pr_stat() output that is noisy
when you are using the python bindings.  This patch provides overrides for these
two functions and the ability for python scripts to optionally turn them off.
With this patch I can silence all of these extraneous print messages from within
my python script.

Signed-off-by: Josef Bacik <jbacik@fb.com>
trace-cmd: use pr_stat for version output

This allows tools linking against the trace-cmd libraries to supress this output
if it is not necessary for them.

Signed-off-by: Josef Bacik <jbacik@fb.com>