freebsd: Increase consistancy with libc implementation

[josephlr]

In #35 @myfreeweb pointed out that the FreeBSD's libc implemenation calls kern_arnd in a loop, and simply retries if the syscall returns a short buffer.

There doesn't seem like a good reason to differ from their implementation, so this PR changes the implementation to be more like the one for Linux getrandom.

[dhardy]

And yet that implementation still has a limit of 256 bytes, so unless someone wants to check up on the kernel source code we should keep the chunking I think.

[newpavlov]

IIUC if we provide buffer larger than 256 bytes, kernel will fill only first 256 bytes and will return 256. Thus we don't need explicit chunking.

[josephlr]

IIUC if we provide buffer larger than 256 bytes, kernel will fill only first 256 bytes and will return 256. Thus we don't need explicit chunking.

This is correct, I also think the the 256 bytes is an implementation detail; I couldn't find documentation for the behavior. In that case, this implementation no longer makes assumptions about how KERN ARND works.

[dhardy]

In that case, 👍 . Over to you @newpavlov