bug#10429: 23.3; Many "user variables" in man.el aren't defined accordingly

[Chong Yidong <cyd@gnu.org>]

Juri Linkov <juri@jurta.org> writes:

>> Perhaps it would not provide much real benefit to expose all of the
>> regular expressions through customize (especially with the way many of
>> the bigger ones are built up from smaller pieces). Everything else,
>> however, would probably work quite nicely.
>
> It would clutter the Customization interface to include all variables.
> But we could include at least whose customization is suggested in docstrings,
> like e.g. the docstring of `man' says:
>
>   Add to `Man-switches' to make this option permanent.
>
> so making `Man-switches' customizable would be reasonable.

I converted the ones which make sense into defcustoms.