perf tools: Add new COMM infrastructure

This new COMM infrastructure provides two features:

1) It keeps track of all comms lifecycle for a given thread. This way we
can associate a timeframe to any thread COMM, as long as
PERF_SAMPLE_TIME samples are joined to COMM and fork events.

As a result we should have more precise COMM sorted hists with seperated
entries for pre and post exec time after a fork.

2) It also makes sure that a given COMM string is not duplicated but
rather shared among the threads that refer to it. This way the threads
COMM can be compared against pointer values from the sort
infrastructure.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Tested-by: Jiri Olsa <jolsa@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-hwjf70b2wve9m2kosxiq8bb3@git.kernel.org


[ Rename some accessor functions ]
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
[ Use __ as separator for class__method for private comm_str methods ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>