Add Lockf POSIX to UNIX targets

[pickfire]

Fixes #1631

[rust-highfive]

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[pickfire]

r? @semarie

[semarie]

about the errors: it seems the constants are already defined for several platforms.
could you try to just keep the lockf function definition ?

please, do not open a new issue, but amend your branch, and push force it (git push --force): it will "update" the current PR.

[pickfire]

@semarie I moved it to another branch then it creates a new issue.

[JohnTitor]

Looks good to me, thanks!