perf sched: Implement the 'perf sched record' subcommand

Implement the 'perf sched record' subcommand that adds a
default list of events, turns on raw sampling and system-wide
tracing and passes off the rest of the command to perf record.

This is more convenient than having to specify the events all
the time.

Before:

 $ perf record -a -R -e sched:sched_switch:r -e sched:sched_stat_wait:r -e sched:sched_stat_sleep:r -e sched:sched_stat_iowait:r -e sched:sched_process_exit:r -e sched:sched_process_fork:r -e sched:sched_wakeup:r -e sched:sched_migrate_task:r -c 1 sleep 1

After:

 $ perf sched record -f sleep 1

Also fix an assumption in the event string parser that assumed
that strings passed in can be modified. (In this case they wont
be as they come from a readonly constant section.)

Signed-off-by: Ingo Molnar <mingo@elte.hu>