Avoid useless /bin/sh hanging in memory
Last statement in surfraw(1) is invoking specific elvi. As long as it do not return (e.g user browses web), shell of surfraw(1) hanged in memory.
With this patch, shell of surfraw(1) uses "exec" to invoke specific elvi, saving one fork() and making sure, that /bin/sh process do not exists longer, then necessary.