perf record: Change switch-output option to take optional argument

perf record: Change switch-output option to take optional argument

Next patches will add --switch-output option arguments, changing the
option to allow that and adding its default value to 'signal'.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Wang Nan <wangnan0@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1483955520-29063-4-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>