Break /etc/bash_completion into /usr/share/bash-completion/{.pre,base,.post}

Break /etc/bash_completion into /usr/share/bash-completion/{.pre,base,.post}
so that the base definitions can be eselected. Rename
/etc/profile.d/bash-completion to have a .sh extension so it's loaded
automatically by /etc/profile. This is okay because it doesn't do anything
until modules have been eselected. All of this makes it possible to
configure bash-completions entirely with eselect instead of needing to add
snippets to one's .bashrc
Package-Manager: portage-2.1.2_rc1-r7