bug#1512: 23.0.60; SPC, TAB during completion do not do word completion, prefix completion

[Drew Adams]

> > Prior to Emacs 23, both regular prefix completion and word 
> > completion were available by default: TAB for the former,
> > SPC for the latter.
> 
> Define "word completion", please,

What SPC does in Emacs ..., 20, 21, 22: `minibuffer-complete-word'.

>From Emacs manual, node Completion Commands:

`<SPC>'
Complete up to one word from the minibuffer text before point
(`minibuffer-complete-word').  <SPC> for completion is not
available when entering a file name, since file names often include
spaces.

And by "prefix completion", I mean what TAB did in Emacs ..., 20, 21, 22.