perf tools: Introduce per user view

The new --uid command line option will show only the tasks for a given
user, using the proc interface to figure out the existing tasks.

Kernel work is needed to close races at startup, but this should already
be useful in many use cases.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
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-bdnspm000gw2l984a2t53o8z@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-record.txt

@@ -58,6 +58,10 @@ OPTIONS
 --tid=::
         Record events on existing thread ID.
 
+-u::
+--uid=::
+        Record events in threads owned by uid. Name or number.
+
 -r::
 --realtime=::
 	Collect data with this RT SCHED_FIFO priority.

tools/perf/Documentation/perf-top.txt

@@ -78,6 +78,10 @@ Default is to monitor all CPUS.
 --tid=<tid>::
         Profile events on existing thread ID.
 
+-u::
+--uid=::
+        Record events in threads owned by uid. Name or number.
+
 -r <priority>::
 --realtime=<priority>::
 	Collect data with this RT SCHED_FIFO priority.