[krbdev.mit.edu #8499] Use getrandom system call on recent Linux kernels

[Greg Hudson via RT]

Recent Linux kernels provide a getrandom system call which can have better 
behavior than reading from /dev/urandom--it can block when the kernel PRNG 
is unseeded, but not when the kernel thinks the acquired entropy pool is 
depleted.  Using a system call is also more efficient and requires less 
from a chroot environment than reading from a device.  Unfortunately, glibc 
does not currently provide a binding to the new system call, so we need to 
use syscall() to access it.

As part of this work, we will rename prng_os.c to prng_device.c, and create 
a new prng_os.c which does not burn a file descriptor.  Downstream 
integrators who do not want the user-space Fortuna PRNG can choose 
prng_os.c for platforms which use a getrandom system call, and 
prng_device.c for platforms which do not.