Add function for command completion for tc in bash, and update Makefile
to install it under /usr/share/bash-completion/completions/.

Inside iproute2 repository, the completion code is in a new
`bash-completion` toplevel directory.

v2: Remove `if` statement in Makefile: do not try to install in
    /etc/bash_completion.d/ if /usr/share/bash-completion/completions/
    is not found; instead, the user can override the installation path
    with the specific environment variable.
Signed-off-by: Quentin Monnet <quentin.monnet@6wind.com>