Expose 'array_length()' macro in <sys/param.h>

[Florian Weimer]

* Ville Voutilainen:

> Why should this be array_length and not __array_length? This is a
> vendor extension, so it should use
> a name that is reserved for such?

In general, we use __ for glibc internals that are not intended for
direct access by users.  In some cases, we have non-__ identifiers that
are redirected to these __ identifiers.

In some cases it may be beneficial to expose vendor extensions under __
names, so that they can be used in strict standard modes, but we are
quite far away from that for C++ at least.

Thanks,
Florian
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