Substitutes a literal GID UID with the UID GID of the current owner of the process.
execl-subuidgid [ -o owner ] prog
execl-subuidgid if { mkdir -p /run/user } chown -R $UID:$GID /run/user
execl-subuidgid -o root if { mkdir /run } chmow -R $UID:$GID /run/user