Enable the use of __attribute__ ((__nonnull__)) unconditionally under OpenBSD,
as has been done for the __sentinel__ attribute, since all toolchains support
it.

Merge OpenBSD specific changes:
- pexecute implementation replaced with the older gcc 2.95 implementation,
as gcc 2.95's collect2 relies upon an implementation behaviour which has
been lost in fu

Merge OpenBSD specific changes:
- pexecute implementation replaced with the older gcc 2.95 implementation,
as gcc 2.95's collect2 relies upon an implementation behaviour which has
been lost in further libiberty updates.
- keep the old hash table interfaces, which are used by gcc 2.95.
- keep the OpenBSD replacement for the md5 interface.
- keep the OpenBSD strl{cat,cpy} and snprinft usage.

Bump libiberty minor version due to the addition of a few new symbols.

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Update libiberty to the version found in binutils 2.17.1 (which still
identifies itself as the gcc 3 version...)

Synch with gcc-3.3.2 version

And the includes are taken from GCC 2.95.pre3 test3