After commit 6dd6a57c60, 3 remaining
builtins were affected by uint8_t overflow: `exit`, `return`, and
`functions --query`.
This commit:
- Moves the overflow check from `builtin_set_query` to `builtin_run`.
- Removes a conflicting int -> uint8_t conversion in `builtin_return`.
- Adds tests for the 3 remaining affected builtins.
- Simplifies the wording for the documentation for `set --query`.
- Does not change documentation for `functions --query`, because it does
not state the exit code in its API.
- Updates the CHANGELOG to reflect the change to all builtins.
This would tell you a function was "Defined in - @ line 1" for every
function defined via `source`.
Really, ideally we'd figure out where the *source* call was, but that'
much more complicated, so we just give a comprehensible message.
