make pg_ctl more friendly

Hi hackers:

I got a basebackup using pg_basebackup -R. After that, I created a restore
point named test on primary, and set recovery_target_name to test,
recovery_target_action to shutdown in standby datadir. I got a failure
startup message after 'pg_ctl start -D $standby_datadir'. I think it is
not a failure, and makes users nervous, especially for newbies.

My thought is to generate a recovery.done file if the postmaster receives
exit code 3 from the startup process. When postmaster exits, pg_ctl will
give a more friendly message to users.