Freebsd: Try getrandom() first

[josephlr]

Depends on #54fixes#35

Implementation is quite similar to the Solaris/Illumos implementation. @myfreeweb PTAL

Edit: Also adds a fill_exact helper method for making repeated OS calls to fill a buffer. This let's us simplify the Linux/Solaris/FreeBSD code, as well as adding some changes useful for #54 and #58

[valpackett]

Looks good, works fine.

[newpavlov]

Maybe it's worth to make this PR independent from #54? I think it will be some time before we will converge our opinions on that one, while changes in this PR look quite straightforward.

[josephlr]

Maybe it's worth to make this PR independent from #54? I think it will be some time before we will converge our opinions on that one, while changes in this PR look quite straightforward.

The main reason for the change is that this implementation uses the fill_exact helper method introduced in #54. I can put those changes in this PR if you want.

[newpavlov]

I do not insist on it, you can leave it as-is. I was simply thinking it may be easier for you, since no syncing with #54 would be needed.

[josephlr]

I was simply thinking it may be easier for you, since no syncing with #54 would be needed.

Good point, I added fff1bde to this PR that adds in the helper function changes.

[dhardy]

Looks good, other than a comment as mentioned.

[dhardy]

Seems odd to me to assume that calling last_os_error is appropriate here given the function name. At least this should be mentioned by function documentation.

[newpavlov]

During removal of std dependency it will have to be replaced with an explicit errno, so I think it's fine to keep it as-is for the time being.

[josephlr]

So I changed the function to be named sys_fill_exact and added a comment to explain the expected semantics of the sys_fill function. @newpavlov is right that this will be replaced with an explicit call to an errno function in #54