perf flamegraph: flamegraph.py script improvements

perf flamegraph: flamegraph.py script improvements

* display perf.data header
* display PIDs of user stacks
* added option to change color scheme
* default to blue/green color scheme to improve accessibility
* correctly identify kernel stacks when kernel-debuginfo is installed

Signed-off-by: Andreas Gerstmayr <agerstmayr@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20210830164729.116049-1-agerstmayr@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>