afs: Write 'afs' to argv[0].

This allows to distinguish the afs process from the main server
process by looking at the ps output. Although there are less hacky
ways to achieve this (prctl(PR_SET_NAME, ...), setproctitle()), the
argv approach has the advantage of working on all supported operating
systems except FreeBSD. We have been doing the argv mangling for a
long time now in the command handlers and it has never caused any
problems, so it should be safe.