perf record: auto detect when stdout is a pipe

This patch gives the ability to 'perf record' to detect when its stdout
has been redirected to a pipe. There's now no more need to add '-o -'
switch in this case.

However '-o <path>' option has always precedence, that is if specified
and stdout has been connected via a pipe then the output will go into
the specified output.

LKML-Reference: <m3ipxo966i.fsf@gmail.com>
Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>