Standardize to using expanded reference codes

[slifty]

In our comments and some errors we sometimes use the code prefixes when referencing the SFTP spec (e.g. SSH_FXP_FOO) and sometimes we omit it (e.g. FOO).

Let's always use the prefixes, in both cases.

This won't impact any functionality but it should make logs easier to connect to the SFTP spec when doing searches.