[gix-tempfile] Revisit connection to signal-hook

[Byron]

#336 was a reminder on how good intentions can end up having undesirable impact which is why it should be put into question if the bundling with signal-hook is the right choice for a plumbing crate.

Despite the ecosystem being centered around signal-hook, it seems clear that other options are possible that shouldn't have to deal with an unnecessary dependency.

It also appears that more complex applications will want their own signal handlers and free reign on how to integrate the cleanup, for reference, git-repository also integrates the cleanup handler directly instead of calling setup().

Revisit the way the cleanup handler is promoted to users of the library to make it reasonably easy to use while avoiding accidental behaviour to emerge.