variables: prefer error-verbose to error_verbose

[Akim Demaille]

Pushed in master only.  But already obsoleted by our desire to stay  
away from Boolean variables.  Yet it allows to stress some of the  
infrastructure, so I pushed it.

        * data/bison.m4 (b4_error_verbose_if): Based on error-verbose
        instead of error_verbose.
        * src/scan-gram.l (%error-verbose): Map the error-verbose
        variable.
        * doc/bison.texinfo: Promote %define error-verbose instead of
        %error-verbose.
        * tests/actions.at: Prefer %define error-verbose to %error-verbose.

0003-variables-prefer-error-verbose-to-error_verbose.txt
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