perf tools: Add time-based utility functions

Add function to parse a user time string of the form <start>,<stop>
where start and stop are time in sec.nsec format. Both start and stop
times are optional.

Add function to determine if a sample time is within a given time
time window of interest.

Signed-off-by: David Ahern <dsahern@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1480439746-42695-2-git-send-email-dsahern@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>