emacs: include first match in address completion

emacs: include first match in address completion

The current code for address completion takes the list of possible
completions (whether generated internally or externally), makes the
first match the initial value for the completion, and puts all the
others (but not the first match) into the possible completions.

This has the nice effect that the <down> key takes you immediately to
the next completion (whereas if the first match were included in the
possible completions it would take you to the first match
again).

However, it has two side effects. First, once you have completed to
the full match you find it says and try completing again you get told
"no match" not "sole completion". Secondly, if you delete some of the
text and try completing you don't get the first match as an option.

This tries to get round most of these problems by including the full
list of possible completions, but with the first match moved to the
very end of the list.