reporter_kbpki: suppress "_mtn" username lookup
reporter_kbpki: suppress "_mtn" username lookup

Emacs on Linux seems to do this.

Issue: KBFS-825

keybase/client,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs