lib/randutils.c: Fall back gracefully when kernel doesn't support getrandom(2).

lib/randutils.c: Fall back gracefully when kernel doesn't support getrandom(2).

The 3.16 kernel is supported until 2020, and various distros have kernels of the same
vintage. It's entirely possible for code built against newer headers to be run against
these kernels, so fall-back to the old “read /dev/{u,}random” method if the kernel doesn'
support getrandom()