Add more cfg aliases

[asomers]

Add cfg aliases for linux_android, bsd, and freebsdlike. Use them in many places, though not everywhere they could theoretically be used.

Fixes #2188

What does this PR do

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[SteveLauC]

Should we also do this for Solaris and Illumos?

cfg_aliases! {
    illumos: { target_os = "illumos" },
    solaris: { target_os = "solaris" },
        
    solarish: { any(solaris, illumos) }
}

BTW, we should document this in probably CONVENTIONS.md (or CONTRIBUTING.md?) so that our contributors can be aware of these aliases and use them in PRs

[Jan561]

Are other config-aliases, like an alias mmsg for

#[cfg(all(
    feature = "socket",
    feature = "uio",
    any(
        target_os = "linux",
        target_os = "android",
        target_os = "freebsd",
        target_os = "netbsd",
    ),
))]

desirable for the future? Or should they just stay platform-related for the time being?

[asomers]

Are other config-aliases, like an alias mmsg for

#[cfg(all(
    feature = "socket",
    feature = "uio",
    any(
        target_os = "linux",
        target_os = "android",
        target_os = "freebsd",
        target_os = "netbsd",
    ),
))]

desirable for the future? Or should they just stay platform-related for the time being?

I think we can add them on an as-needed basis.

[SteveLauC]

Another place where we can use solarish

[SteveLauC]

Should be: solarish

[SteveLauC]

I just checked that the changes made in this PR are good.

But there are also still cfgs that haven't been updated, we should handle them as well, but not in this PR, instead, we should probably split the remaining job into several PRs, which will make code review easier.

For #2188, let's turn it into a tracking issue, once the tasks in it are all done, then we can close it.

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For the requested changes, I have committed 2 with the GitHub UI, please ignore the left ones and handle them in other PRs