x/sys/unix: add Mremap

[rsc]

syscall provides a type-safe Mmap and Munmap but is missing Mremap. We should add it.
See #58625 (comment).

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[mdempsky]

mremap isn't in POSIX, so not all UNIXes support it. In particular, it looks like at least darwin and openbsd don't implement it.

[willfaught]

I thought syscall was frozen? Don't these changes belong in x/sys now?

[ianlancetaylor]

Yes, looks like this support is copied into x/sys/unix. You're right, that is what we should change. Thanks.

[rsc]

We do use syscall.Mmap from within std, so it's OK to update if necessary. That said, std doesn't need Mremap so adding it in x/sys/unix instead is fine.

[rsc]

Marking as likely accept for x/sys/unix. We can change syscall if we need it for something in the main distribution but we probably don't need it otherwise.

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group