Rename -W to -Wextra ?

[Joe Buck]

Phil writes:
> - recognize both -W and -Wextra
> - for --help output and in the manual, only describe -Wextra
> - document all the little extra things that -W[extra] currently does
> 
> i.e., print no deprecation warnings for -W, leave it in there, but don't
> advertise it anymore.  It's too confusing.

Pretty much OK, but the manual should be complete.  Perhaps -W should
be mentioned at the end as a backward-compatible synonym for -Wextra.

> I can do the above, no problem.  Right now I do not have the time to split
> out -Wextra into 20 separate options, or to split out extra_warnings into
> 20 separate variables.  However, the updates to the manual should help
> anyone who wants to do so.

I wouldn't be surprised if there are only two or three of the -W warnings
that people would like to be able to invoke seperately.