Backport improvements from `packets2` to `packets1`

[dmajda]

When creating the packets2 package with packets1 as the base, the code was improved in some places. These improvements should be backported to packets1 at some point. This issue contains a list of such instances and should serve as a reminder.

List of improvements

• Better ordering in packets2/packets2.go when compared to packets1/packets1.go (noted here).
• Using a constant instead of hardcoded header length in packets2/auth.go (noted here).
• Using RC_CONGESTION as ReturnCode/ReasonCode value instead of RC_ACCEPTED in PUBACK, REGACK, and SUBACK tests (see here).

[mprymek]

Separate issue:
"the error handling in NewPacketWithHeader changed" (#54 (comment))

Addressed in #63