timeval: Add new "backtrace" appctl command.

timeval: Add new "backtrace" appctl command.

Often, it can be quite difficult to debug performance issues in
Open vSwitch.  Typically one needs to run something like gprof, but
that requires rebuilding and installing on the affected system
which is often problematic.  This patch adds a light weight
profiling solution which can be used in these situations.  The
ovs-appctl backtrace command prints out backtraces taken at 100
millisecond intervals over a 5 second period of time.  It is
currently only supported on systems which have the execinfo library
and enable time caching.

Signed-off-by: Ethan Jackson <ethan@nicira.com>