File-staging failures that cannot succeed are marked permanent

What

Staging is how files are copied into a session, for example in cloud sessions. Failures that will never succeed on a retry are now tagged permanent: true, where before they only reported failure and an error message.

• HTTP 4xx errors are permanent, except 408 (timeout) and 429 (too many requests).
• These file-system errors are permanent: ENAMETOOLONG, ENOSPC, EDQUOT, EFBIG, EISDIR, ENOTDIR, EINVAL, ELOOP, ENOTEMPTY, and STAGE_PARENT_ESCAPE (a file trying to land outside its allowed folder).
• Fetches blocked by a gate are permanent. The "list gated" error used to print the wrong value as its reason; it now shows the real reason (list gated: ${r.reason}).
• The record of a completed staging gains a replaced field, and overwriting a file with different contents is logged as a warning.

Why

Callers can stop retrying uploads that cannot work, such as a full disk or a name that is too long, instead of looping. A gated list now explains itself with a readable reason.