arch: kill dirmngr only

arch: kill dirmngr only

Using fuser to identify processes using a path doesn't work as I wanted:
it relies on the filesystem mounted there. The outcome is that when
using "-t directory" it will try to kill all processes using that
filesystem rather than processes using files under that path.

Even by bind-mounting root so it's always a mountpoint, it doesn't work.
So instead of killing everything, pinpoint what process pacman/pacstrap
left running and kill only that one.

Right now this means telling dirmngr to exit.