%name-prefix is broken.

* src/files.c (spec_name_prefix): Initialize to NULL, not to "yy".
Adjust all dependencies.
* tests/headers.at (export YYLTYPE): Strengthen this test: use
%name-prefix.
Renaming yylval but not yylloc is not consistent.  Now we do.
* src/bison.simple: Prefix yylloc if used.
* doc/bison.texinfo (Decl Summary): Document that.