Silence Bison deprecation warnings

Bison >=3.0 issues warnings about

    %name-prefix="base_yy"

instead of the now preferred

    %name-prefix "base_yy"

but the latter doesn't work with Bison 2.3 or less.  So for now we
silence the deprecation warnings.
Save pg_stat_statements statistics file into $PGDATA/pg_stat directory at shutdown.

187492b6c2e8cafc5b39063ca3b67846e8155d24 changed pgstat.c so that
the stats files were saved into $PGDATA/pg_stat directory when the server
was shutdowned. But it accidentally forgot to change the location of
pg_stat_statements permanent stats file. This commit fixes pg_stat_statements
so that its stats file is also saved into $PGDATA/pg_stat at shutdown.

Since this fix changes the file layout, we don't back-patch it to 9.3
where this oversight was introduced.