Add support for getrandom(2), add type check on file descriptor

Add support for getrandom(2), add type check on file descriptor

Fix to_read, throw exception if syscall fails

Fixes thanks to feedback from sarnold at ##crypto on freenode

Correction on error conditions

Remove dead code (thanks @defuse)

It turns out getrandom can take >256, getentropy refuses.

Better semantics

Thanks @defuse for catching my silly mistake here

Cast to size_t to be explicit

Let's simplify the logic a bit

Let's be consistent; define everything before we do any logic

Continuously check that the file descriptor is still a valid one

Add device type check on fd initialization