Adopt XDG spec or add option to overwrite histfile location (.drgn_history)

[septatrix]

It would be great if drgn could follow the XDG spec and place its history file somewhere under ~/.local/ or at least offer an option to overwrite the default location, e.g. like $HISTFILE for bash

[osandov]

This sounds reasonable.

Apparently the XDG spec didn't have a standard place to put history files when I added ~/.drgn_history back in 2018 (0a089e6). But the May 2021 revision added XDG_STATE_HOME for that purpose.

Let's change it to $XDG_STATE_HOME/drgn/history, where $XDG_STATE_HOME defaults to ~/.local/state per the spec. In terms of migration, if we don't find $XDG_STATE_HOME/drgn/history but we do find ~/.drgn_history, we can move it to the new location, at least for a few releases.

[brenns10]

As a note, I believe the contrib/ptdrgn.py script has its own history file which should probably get the same treatment. If it's not part of this I can follow up with it too.