Rename Process kill methods to signal [by @jan-zajic]

[oprypin]

Rename Process#kill(Signal) to Process#signal(Signal) that we can eventually implement on Windows (if needed) with a deprecation notice. The name kill is confusing as it can mean two things - send signal to process, or send signal kill (9) to process when argument omitted. But currently default behavior of kill method without providing signal argument is to send term signal to process (simulates posix behavior of kill command, but we need to remove POSIXisms of apis to become really portable)

Closes #8642. I'm re-creating the PR to be able to resolve outstanding comments.

This probably should be squash-merged, with the above message.

[straight-shoota]

Thanks for picking this up 👍
I've just some minor wording suggestions, to be a bit more precise (and also align with the existing documentation).

[straight-shoota]

@oprypin Why did you remove the example which included specs for terminate?

[oprypin]

@straight-shoota , #8642 (comment)

Funnily enough, that spec is exactly the same as the one preceding it, as kill and terminate are the same thing for now

[straight-shoota]

I see. We could still run #terminate in a spec, though. Even if it's not possible / very difficult to test it behaves correctly. But we should make sure it compiles and runs.

[oprypin]

(make sure to squash when merging)