Add lockf

[pickfire]

Fix #1631

[rust-highfive]

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[semarie]

as lockf() is POSIX function, I am suspecting that it is existing on several unix too (if not all). could you try to move your definition from linux_like to unix directory in order to see what CI will do ? (it should cover most of the supported platform)

Note I have checked that OpenBSD has it, and have same constants too.

[pickfire]

@semarie Isn't lockf not a POSIX function? The man page I found is 3 not 2p.

Still I will send another patch.

[semarie]

the POSIX definition is here.

regarding man page section, it is really distribution specific, so I can't really comment :-)

[pickfire]

Superseded by #1708