What is the “or” in contractor, signor, terminator, or conductor considered? A suffix?
Sure, we’ll go with suffix.
Very much a suffix.
And the dudes who stick it on there are suffixors. 
Both -or and -er are related to the word wer (preserved in “werwolf”), which means man. It is cognate with Latin vir and Irish fear. So to all those who say “server” is a gender-neutral substitute for “waiter / waitress,” I say that there is simply no escaping the inherent sexism of the Indo-European languages.