Backport various commits
    
    97df75d7bd13ae9843fb55dd7285bf113adc2bd2: socket: pass socket FDs to all ExecXYZ= commands but ExecStartPre=
    87c4c199a590b26889c5158906925f317a87ce19: core: Serialize both pid and pidfd to keep downgrades working
    e3783068c6dc1d5199c30c6ee4835f16ad92d3f9: core/mount: if mount is gone eventually, consider it success
    8e94bb62a5c1309c56c57e0a505aae13a2ac5f4f: core/mount: if umount(8) fails but mount disappeared, assume success