Update bash completion for Debian.

This improves the completion, as suggested in
<https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=782169

>,
and moves the completion from `/etc/bash_completion.d/mosh` to
`/usr/share/bash-completion/completions/mosh` as recommended by
lintian in its `package-install-into-obsolete-dir` check.

Closes #628.

Signed-off-by: John Hood <cgull@glup.org>