whereis: use commands for Bash completions

Currently, the Bash completions for `whereis <TAB>` gives
`whereis file` ("file" is not a placeholder here, it literally expands
to that). This fixes that by passing the `-c` flag to `compgen` to
request completions for command names.

Signed-off-by: Smitty van Bodegom <me@smitop.com>

bash-completion/whereis

@@ -22,7 +22,7 @@ _whereis_module()
 			return 0
 			;;
 	esac
-	COMPREPLY=( $(compgen -W "file" -- $cur) )
+	COMPREPLY=( $(compgen -c -- $cur) )
 	return 0
 }
 complete -F _whereis_module whereis