Forward port 2.x code quality improvements to 3.x

[rdmark]

There are a handful of minor improvements that were made in 2.x that should be ported to 3.x, e.g. this commit https://github.com/Netatalk/netatalk/commit/bda82747ba63bebe72e202187c6f4036f71fbce2

Examples include safety checks in afppasswd.c

[rdmark]

I was able to port most relevant patches cleanly in https://github.com/Netatalk/netatalk/pull/952

Two patches I'm holding back for now because they don't apply cleanly to main, and some deeper reading is required to make sure there are no side effects:

https://github.com/Netatalk/netatalk/commit/34c6c306dcccfa3c3608c745b949e74cdb36d916 https://github.com/Netatalk/netatalk/commit/9a75cb899f366cd364e1921dab3ce9c229f56725

[rdmark]

The former now done with https://github.com/Netatalk/netatalk/pull/1023

The latter is just cleanup of unused variables. That can be done at a later time for the codebase as a whole.