Fully rename snmptrapd to navtrapd internally

[lunkwill42]

#2926 renamed the snmptrapd command line program to navtrapd, but the change is only in how the binary is named. Internally, the Python module is still named snmptrapd.

            Merge this now, do the internal rename(s) in a later patch-release so that we can get feedback on the new externally visible name as soon as possible.

Originally posted by @hmpf in #2926 (review)