mirage-crypto-rng.unix: add Hurd support

[pinotree]

Implement raw_getrandom() for Hurd, using getrandom() as available in GNU libc.

[hannesm]

Thanks @pinotree. Is there a path to test such a Hurd system in CI (GitHub actions)? Is the C define of Hurd really __GNU__, isn't there something like __HURD__ as well? (__GNU__ looks pretty coarse, and other systems may define this as well).

[hannesm]

Also, can't we fold that into the __linux ifdef (starting at line 8), since the actual code is the same.

[pinotree]

Is there a path to test such a Hurd system in CI (GitHub actions)?

Unfortunately not.

Is the C define of Hurd really __GNU__, isn't there something like __HURD__ as well? (__GNU__ looks pretty coarse, and other systems may define this as well).

Considering GNU/Hurd used to be considered "the GNU system", it got __GNU__ as identifier; it has also __gnu_hurd__. There is no risk that other OSes will use __GNU__ now, as it would clash with GNU/Hurd.

[1] https://github.com/cpredef/predef/blob/master/OperatingSystems.md#gnu-aka-gnuhurd

Also, can't we fold that into the __linux ifdef (starting at line 8), since the actual code is the same.

Sure. Sometimes sharing code branches with other OSes is helpful, sometimes only adds extra maintenance burder.

[hannesm]

Thanks. Also thanks for the link to the predef.