sdexec: do not clean up failed units

[chu11]

Per discussion in #5956, it may be useful at times to not cleanup failed systemd units for post analysis later.

The way this was handled in the old libsdprocess was through:

    /* In sdprocess, we require the systemd unit to exist until the                                                                            
     * user cleans it up with sdprocess_systemd_cleanup().  This                                                                               
     * ensures consistent behavior in a number of functions.  For                                                                              
     * example, a function like sdprocess_wait() can be called                                                                                 
     * multiple times.  Therefore, we set RemainAfterExit to true                                                                              
     * for every process we start.                                                                                                             
     */                                                                                                                                        
    if ((ret = sd_bus_message_append (m,                                                                                                       
                                      "(sv)",                                                                                                  
                                      "RemainAfterExit",                                                                                       
                                      "b",                                                                                                     
                                      true)) < 0) {                                                                                            
        set_errno_log (sdp->h, ret, "sd_bus_message_append");                                                                                  
        return -1;                                                                                                                             
    }      

This apparently keeps all units around. This would pile up old units quickly and not be good, so perhaps we could make this a per-job configuration, in case we need to debug a very specific job that is failing in a unique way. Then after that job was done we'd clean up the successful units. Or somewhere in the sdexec module it would do the cleanup of successful units.

[garlick]

We do set RemainAfterExit on all units now:

https://github.com/flux-framework/flux-core/blob/master/src/common/libsdexec/start.c#L302

As I recall, successful units do not remain but failed ones do, until ResetFailedUnit is called:

https://github.com/flux-framework/flux-core/blob/master/src/modules/sdexec/sdexec.c#L283

[chu11]

~~Ahh, hmmm then why the failed one from #5956 didn't stick around is a mystery (although it's possible I did not use quite the right systemctl runes ...)~~ neverind, I misread your prior comment.

So perhaps we shouldn't called ResetFailedUnit on some special config.