runtime: use XDG_CONFIG_DIR for the userconf path

[mboersch]

This changes the config path from ~/.gnuradio to ~/.config/gnuradio. On windows this maps to %APPDATA%/.config/gnuradio.
This was requested in #3631.

Description

Adhering to the XDG specification help reduce clutter of custom dotfile locations in $HOME...

Related Issue

Fixes #3631

Which blocks/areas does this affect?

runtime

Testing Done

CMake build and ctest.

Checklist

• I have read the CONTRIBUTING document.
• I have squashed my commits to have one significant change per commit.
• I have signed my commits before making this PR
• My code follows the code style of this project. See GREP1.md.
• I have updated the documentation where necessary.
• I have added tests to cover my changes, and all previous tests pass.

Not sure how I should document this or where to even start. Dito testing :-)

[willcode]

I like the idea of having the GR conf file use the system scheme. We've got an awful lot of people out there with the conf file in the current location, so I feel like some sort of transition is needed. That might involve the old location having priority, or copying the file to the new location (and removing the old one?) or something else. Then there's the user "where did my file go?" problem. Needs some thought.

[marcusmueller]

@mboersch would you please run clang-format on your changed files?

@willcode I think for main a change where ~/.gnuradio doesn't have priority (but is still found, if XDG_CONFIG_HOME doesn't contain config) is OK, even desirable!

The backport to maint-3.10 (which sadly is explicitly necessary, if we want to do it, because these functions are public API…) would probably want to not do anything "automatically surprising" about the file(s)'s location, fully agreed. But we could print a nice log entry, and I wouldn't mind adding a GRC button that does move the configuration, abusing, say, the "Help" menu.

[willcode]

Things that are main-only may never see the light of day. Don't want to waste peoples' time. Also, this is an area where different behavior between main and releases could really confuse people who run multiple versions.

[mboersch]

@mboersch would you please run clang-format on your changed files?

Done.

Some backwards compatibility seems reasonable -- even though that code would probably live forever.

I can try add a lookup to the old .gnuradio to the lookup list for the userconf path.
I'm not sure if I should put the lookup / filesystem checks into the sys_path.cc -- seems a bit odd to have the PATH code do filesystem checks.
FWIW, introducing a xdg_userconf_path and a legacy_userconf_path there, and have userconf_path check if the legacy directory exists may work. Should I go for it?

[dkozel]

Throwing in, a change to only using the XDG path is reasonable for GR 4. I agree backwards compatibility/transitioning is needed or we'll substantially confuse folks doing upgrades.

[willcode]

Marked with "API Change" so we track that we need to think of a way to make this backward compatible.

[willcode]

Setting as Draft until backward compatibility is worked out.