Reorganize .gitignore to make better use of its features

It's possible to have a very few rules for some directories and trust
that other patterns further along will take care of whatever is left.
.gitignore should therefore be loosely organised from least generic to
most generic, allowing things like this:

    # Keep any file with extensions, such as foo.c, bar.h, ...
    !/dir/*.*

    # ....

    # Remove all object files
    *.o
    *.obj

With this change, we implement some very generic rules for what will
and will not be ignored in the fuzz subdirectory, and truse that
patterns later on (such as *.o, *.obj, *.exe) will take care of
everything we didn't specifically specify for the fuzz subdirectory.

Reviewed-by: Rich Salz <rsalz@openssl.org>