unix: add listns syscall introduced in Linux 6.19

[halfcrazy]

Add support for the listns(2) system call available since Linux 6.19.
The system call enumerates namespace IDs with optional filtering by
namespace type and owning user namespace.

New types:

• NsIdReq: namespace ID request structure (with my_ns_id_req to
  work around cgo -godefs limitation with anonymous nested structs)

New constants:

• NS_ID_REQ_SIZE_VER0, LISTNS_CURRENT_USER
• TIME_NS, MNT_NS, CGROUP_NS, UTS_NS, IPC_NS, USER_NS, PID_NS,
  NET_NS

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

This PR (HEAD: 870d3cc) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/sys/+/781686.

Important tips:

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[gopherbot]

Message from 朱彦:

Patch Set 2:

(1 comment)

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