getrandom(2) support for getentropy_linux

This enables support for the new getrandom(2) syscall in Linux 3.17.

If the call exists and fails, return a failure in getentropy(2) emulation as
well. This adds a EINTR check in case the urandom pool is not initialized.

Tested on Fedora Rawhide with 3.17rc0 and Ubuntu 14.04

ok deraadt@