perf record: Allow specifying a pid to record

Allow specifying a pid instead of always fork+exec'ing a command.

Because the PERF_EVENT_COMM and PERF_EVENT_MMAP events happened before
we connected, we must synthesize them so that 'perf report' can get what
it needs.

[ Impact: add new command line option ]

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <20090515015046.GA13664@ghostprotocols.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>