DragonFlyBSD defines __nonnull incompatibly with glibc,
so avoid the use of __nonnull in Gnulib code.
Problem reported by Gavin Smith in:
https://lists.gnu.org/r/bug-gnulib/2021-03/msg00066.html
* lib/cdefs.h (__attribute_nonnull__): Rename from __nonnull.
All uses in Gnulib changed. There should be no need to change
glibc code that is not shared with Gnulib.
(__nonnull): New macro, defined in terms of __attribute_nonnull__,
and which can be used in glibc code that is not shared with Gnulib.