remove strndupa to build on musl

[mischief]

musl lacks strndupa, and we have attribute cleanup macros, so use them instead.

[keszybz]

I don't think this makes sense. strndupa is a simple function that is used everywhere. So it's approximately a thousand times easier to just implement it once in the c library than to modify every project that uses it. Just fix musl.

The problem with this patch is that strdup can fail. Which means that some error handling needs to be added. And with the error handling, the code is much more verbose and complicated.