perf machine: Introduce find_thread method

There are cases where we want just to find a thread if it exists
already, so provide a method for that.

While doing that start moving 'machine' methods to a separate file.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-8wpzqs9kfupng6xq8hx6lnxa@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/thread.h

@@ -3,6 +3,7 @@
 
 #include <linux/rbtree.h>
 #include <unistd.h>
+#include <sys/types.h>
 #include "symbol.h"
 
 struct thread {
@@ -22,6 +23,7 @@ struct thread {
 
 struct machine;
 
+struct thread *thread__new(pid_t pid);
 void thread__delete(struct thread *self);
 
 int thread__set_comm(struct thread *self, const char *comm);