Find usermountN and set FUSERMOUNT_PROG if not set

[probonopd]

This approach reduces tricky patching of upstream code and exports FUSERMOUNT_PROG instead.

Closes #15, #16, #31, #32, #35, #36, #37

[github-actions]

Build for testing:
artifacts
Use at your own risk.

[probonopd]

Not quite just yet... This happens when I move fusermount and fusermount3 to fusermount98 and fusermount99, respectively:

Probably because trying to execvp it without arguments fails. Hence adding --version.

[github-actions]

Build for testing:
artifacts
Use at your own risk.

artifacts.zip

[TheAssassin]

Haven't had a chance to check the find fusermount function fully yet. But the approach itself looks solid and the rest of the code, too. Please see my comments.

[probonopd]

So far it works on NixOS by default, which is an improvement over what we had without this PR.
It finds and uses /run/wrappers/bin/fusermount3.

However, as soon as one starts experimenting with

sudo mv /run/wrappers/bin/fusermount /run/wrappers/bin/fusermount98
sudo mv /run/wrappers/bin/fusermount3 /run/wrappers/bin/fusermount99

it does not use these but uses /run/wrappers/bin/fusermount3, and as a result we get fusermount3: mount failed: Operation not permitted. And exporting FUSERMOUNT_PROG manually seemingly gets ignored.

[github-actions]

Build for testing:
artifacts x86_64
artifacts i686
artifacts armhf
artifacts aarch64
Use at your own risk.

artifacts x86_64.zip

[probonopd]

Works on NixOS. 👍
Set FUSERMOUNT_PROG_DEBUG=1 for additional debugging output.

[TheAssassin]

I cleaned up the branch and fixed all remaining issues. I'm not a fan of that search function (too complex and using outdated API), but then again, it works fine for now and it doesn't leak.

We still need the patches to FUSE to support the env var. I changed their behavior to demand this input value (which is set by the runtime usually but can be overwritten by the user if needed, albeit we do not want to document any of this). There's also a new debug feature for our FUSE patches. It is what it is. This is the best possible version.

@probonopd please open an issue around upstreaming our patch, possibly with some additional modifications.

[probonopd]

Since upstream does not accept feature requests anymore but PRs, we'd need to turn

https://github.com/AppImage/type2-runtime/blob/e32d17652038e95210abc8b0334ba52da31434a4/patches/libfuse/mount.c.diff

into something suitable for a PR, and I think for that we'd need to make that environment variable optional rather than mandatory.

[probonopd]

"Pull request authors can't approve their own pull request", but I am happy with this and am advocating for it to be merged. Thanks @TheAssassin

[TheAssassin]

we'd need to turn

https://github.com/AppImage/type2-runtime/blob/e32d17652038e95210abc8b0334ba52da31434a4/patches/libfuse/mount.c.diff

into something suitable for a PR

Well...

@probonopd please open an issue around upstreaming our patch, possibly with some additional modifications.