Fish integration: make sure we're not in login mode before sourcing files

[ElectreAAS]

I believe this resolves #5761.

If I understand correctly this comment by a fish maintainer, the standard shells don't read .bashrc or equivalent on login shells. However fish does, and the comment above advises to guard most of the file behind a if not status is-login (or status is-interactive, depending on context).

I apply this advice here.

[ElectreAAS]

Sorry for the messy PR, I wanted to get this one out of my system.
I don't really like discarding the errors with source X >/dev/null 2>/dev/null; or true, if there is a problem in some code called by your init script, you should be aware of it and probably fix it.
Just my opinion though, I could restore that bit

[ElectreAAS]

Note for anyone that had the issue in the first place: your terminal app (mine is gnome terminal for instance) might be set up to launch the same shell as your login shell. This is convenient when you chsh -s, because it auto updates your terminal, but it will bug out here and maybe in other places. Search in the settings for something like "start command line as login shell" and untick it.

[kit-ty-kate]

@ElectreAAS #5841 was merged. Would you be able to rebase your work on top of master? I can do a review afterwards

[ElectreAAS]

I just rebased, it should be compatible now.
Upon reflexion, this PR does 4 things that are almost disjoint:

• refactor the source function to be (100% sujective) more elegant (242bc11)
• Make sure the file exists before sourcing it in ALL SHELLS (b27cf91)
• Don't silence errors with > /dev/null in ALL SHELLS (f7e893a)
• Check that we're not in login mode before sourcing in FISH ONLY (2de9dc3)

I've come to the conclusion that the last point isn't actually desired. Since fish's documentation now clearly warns about doing this, maybe we can roll with the system's assumption that the login shell does not read .bashrc & equivalents.

[rjbou]

I'm reworking tests added in #5036 to be able to integrate them in that PR, like that we can see and track the changes.

[rjbou]

Changelog is missing entries, e.g. changes of script lines

[ElectreAAS]

Should be good now, I'll let you be the judge

[rjbou]

I've added an init scripts test, from #5036 (discovered while setting it up that fish on windows is broken, see #5854). I've also added for each commit the changes in the test file.

[ElectreAAS]

Closing this PR as it was split into #5864, 5865 & 5866.