Support rr release command

[user202729]

Motivation: sometimes it is desirable to make rr not track a subprocess spawned by fork+exec.

(see linux - Support in rr-debugger for not recording a subprocess? - Stack Overflow )

I think there could be two ways

• define rr release <cmd> [args...] command that does something equivalent to the following

   if [[ "$RUNNING_UNDER_RR" == 1 ]]; then
   	rr record --nested=release env --unset=RUNNING_UNDER_RR "$@"
   else
   	"$@"
   fi

• make a release-on-exec configuration that can be passed to rr record that releases a process on fork+exec. (ideally without affecting other things such as create new thread, fork without exec i.e. process spawn worker process, etc.)

The second would be more convenient, but the first would be more flexible, assume the user can modify and recompile the process.