tab completion rewritten

[Yoshinori K. Okuji]

I refactored the tab completion. It is still not friendly with white space in 
filenames, but this is somehow acceptable.

When I worked on this, I found that BSD disklabel support is broken, although 
it should have worked in PUPA. I don't know why it is broken. For now, BSD 
support is not my priority, so I myself won't investigate this problem in 
near future. If you are interested in *BSD, please take a look at it.

Next, I will fix some buggy behaviors of the entry editor and add more useful 
features.

Okuji