bash-completion: make sure that "lastb" actually completes

bash-completion: make sure that "lastb" actually completes

Building util-linux logs a warning on Gentoo:

```
 * Problems with installed bash completions were found:
 *
 * 	lastb: incorrect name, no completions for 'lastb' command defined.
 *
 * For more details on installing bash-completions, please see:
 * https://wiki.gentoo.org/wiki/Bash/Installing_completion_files
```

A bash-completion file is usually composed of two parts:
- a function that knows how to produce completions
- a specification of which command should be completed

In cases where multiple commands have the same options, e.g. commands
that are symlinked to each other, it is common to symlink the completion
files as well. This shares the first part. But the second part cannot be
shared, it must be implemented twice by specifying two different
commands to be completed.

"lastb" had a symlinked completion, but when the completion system
processes /usr/share/bash-completion/completions/lastb, it would only
define a completion for "last".

Define both completions in one file, so that sourcing it works
regardless of command used.