unix/linux: support AT_HANDLE_{FID,CONNECTABLE,MNT_ID_UNIQUE}

[cyphar]

AT_HANDLE_FID and AT_HANDLE_CONNECTABLE are just plain flag defintions.

However, AT_HANDLE_MNT_ID_UNIQUE requires some special handling because
it indicates that the mount ID pointer argument is actually a uint64.

Unfortunately, providing this information to the caller would require
changing the function signature (and a quick code search on GitHub found
several examples where this would break existing programs) so we instead
add a new function OpenByHandleAt64 that can return a uint64 mount ID.

Because it makes little sense to return truncated unique mount IDs,
AT_HANDLE_MNT_ID_UNIQUE is silently masked in the legacy OpenByHandleAt
wrapper. Note that passing this flag value to OpenByHandleAt before this
patch would actually cause stack corruption, so this will not regress
existing working programs (as such programs are already broken).

Fixes golang/go#78742

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Patch Set 1:

(1 comment)

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Message from Ian Lance Taylor:

Patch Set 2: Hold+1

(1 comment)

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Message from Aleksa Sarai:

Patch Set 3:

(1 comment)

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