[dwisiswant0]

When an error had no kind, Kind() assigned ErrKindUnknown to e.kind
before returning it, so the getter wrote to its receiver. An *ErrorX
is often a package-level sentinel read from many goroutines, and
concurrent Kind() calls on one raced on that interface field. The
write also changed the error: parseError copies the kind into every
error that wraps the sentinel, so after one Kind() call those errors
reported unknown-error instead of no kind. Errors given a kind with
SetKind never reached the assignment, which is why they did not race.

Return the fallback without storing it, as Kind() did before bb8871e,
and document on ErrorX that only its setters modify it, so the other
methods stay safe to call on a shared error.

fastdialer calls Kind() on every dial error it returns, so the dial
errors it leaves unclassified used to carry ErrKindUnknown and now
carry no kind, which GetErrorKind reports as an empty kind (#827).
For those errors, GetErrorKind and the kind field in nuclei's error
log change from "unknown-error" to "", IsKind(err, ErrKindUnknown)
returns false, and GetAllErrorKinds stops listing ErrKindUnknown
next to a kind it detects. IsKind against any other kind returns
what it did before.

Fixes: #826
Refs: #827