perf trace: Read thread's COMM from /proc when not set

We get notifications for threads that gets created while we're tracing,
but for preexisting threads we may end not having synthesized them, like
when tracing a 'perf trace' session that will use '--pid' to trace some
other thread.

And besides we should probably stop synthesizing those records and
instead read thread information in a lazy way, i.e. just when we need,
like done in this patch:

Now the 'pid_t' argument in 'perf_event_open' gets translated to a COMM:

  # perf trace -e perf_event_open perf stat -e cycles -p 31601
     0.027 ( 0.027 ms): perf/23393 perf_event_open(attr_uptr: 0x2fdd0d8, pid: 31601 (abrt-dump-journ), cpu: -1, group_fd: -1, flags: FD_CLOEXEC)
                                                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= 3
^C

And in other syscalls containing pid_t without thread->comm_set at the
time of the formatting.

Cc: Adrian Hunter <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Milian Wolff <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Wang Nan <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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