perf record: We should fork only if a program was specified to run

IOW: Now 'perf record -a' works, this was a bug introduced in:

856e9660


"perf record: Properly synchronize child creation"

Also fix the -C usage, i.e. allow for profiling all the tasks in
one CPU.

Reported-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1261957026-15580-1-git-send-email-acme@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>