Use unsafe callbacks

[highjeans]

Description

This PR changes the callback function type to be unsafe rather than safe in (i believe) all functions that need a callback function, as per the discussion in #2198.

Closes #2198

Sources

As no APIs were changes (other than making the callback type unsafe), and safe vs unsafe functions are not documented in the linux documentation/header files, there are no sources.

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[rustbot]

r? @tgross35

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[rustbot]

Some changes occurred in the Android module

cc @maurer

[tgross35]

These look good to me, but there seem to be a few more cases. Would you mind getting those as well? https://github.com/search?q=repo%3Arust-lang%2Flibc%20%2F%3A.*fn%5C(%2F&type=code for a rough search, e.g. the aix module, solaris, espidf, and a few others.

We can't backport this since it is breaking unfortunately, but it will be great to have for 1.0:

@rustbot label +stable-declined

[tgross35]

@rustbot author for the request above

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.