* fix: use single-line response when UIDL has an argument
The test changes included here don't necessarily deal with the fact that
the response used to be multi-line: the failure wouldn't occur during
the `c.Uidl()` calls, but rather on the next one as the client would
still have data from the server to receive, causing a parsing error like
so:
pop3_test.go:103: strconv.Atoi: parsing "unique-id": invalid syntax
The server now correctly replies with a single line response when an
argument is passed, as required by [the spec][1]
[1]: https://www.rfc-editor.org/rfc/rfc1939.html#page-12
* fix: UIDL accepts at most one argument
- Updated error handling to use the error return value for resource closures in tests and functions, ensuring proper error reporting.
- Replaced direct calls to `Close()` with deferred functions that handle errors gracefully.
- Improved readability by using `strings.ReplaceAll` instead of `strings.Replace` for string manipulation.
- Enhanced network connection handling by adding default cases for unsupported network types.
- Updated HTTP response handling to use the appropriate status codes and error messages.
- Removed unused variables and commented-out code to clean up the codebase.
https://datatracker.ietf.org/doc/html/rfc1939#page-6
> If an argument was given and the POP3 server issues a positive response with a line containing information for that message. This line is called a "scan listing" for that message.
