sys::prctl: Adding set_vma_anon_name.

[devnexen]

to set a name for an anonymous region for Linux/Android.

What does this PR do

Checklist:

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

[devnexen]

Seems that there is no explicit way to check if this option is set or not?

No and in this case it does not really matter, this feature is more the "icing of top of cake" kind, so if it does not work it s just a no-op :)