Lower some WARNS from 6 to 5 to eliminate redundant redeclaration errors

Lower some WARNS from 6 to 5 to eliminate redundant redeclaration errors
on yylex().  This allows stable-8 to build on machines running 10 or later
(with a newer yacc that emits its own declaration) and also keep building
on older machines with a yacc that doesn't do so.

This is a direct commit to stable-8 because it fixes a problem that doesn't
exist on later branches.

git-svn-id: svn://svn.freebsd.org/base/stable/8@279041 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f