using modules in completion functions

[Oliver Kiddle <opk@u.genie.co.uk>]

While looking through the manual, I noticed the disable-stat style is
used for cvs completion when deciding whether to load the stat module.
For completing file descriptors, I had taken a different approach where
the stat module was used if already loaded but if not, ls is used. I
would expect that it will be a fairly common situation that we will
have where a completion function can make good use of a module but can
cope without it and we have to decide whether or not to use it. It
would therefore be a good idea if we agreed on a common way to handle
this.

What I would suggest is a style named something like use-module which
can be set to one of three values which would correspond to:
1. always use the module, loading it if necessary
2. never use the module
3. use the module but only if it is already loaded
where the third should probably be the default

The name of the module could be placed somewhere in the context. Where
would it make sense to put it - the argument, as a sub-command or
elsewhere? If it can't go in the context, I suppose we could have
separate styles such as use-module-stat but that wouldn't be so good in
my opinion.

Oliver