Rename __unused fields to __glibc_reserved.

Rename __unused fields to __glibc_reserved.

__unused is often defined to __attribute__((unused)) in BSD
sources and furthermore libstdc++ testsuite uses it as a macro
to verify libstdc++ headers don't use __unused identifiers.
In ~2012 glibc headers have been cleaned up, but some new
uses of __unused have reappeared (s390 fenv.h already many
years ago, the rest last November).

Reviewed-by: Florian Weimer <fweimer@redhat.com>