crypto_perror is a no-no, since an operation can set more than one
error.
Also, fix a bug in the unix crypto_seed_rng: mixing stdio with
/dev/urandom is a bad idea, since fopen can make all kinds of weird
extraneous syscalls (mmap, fcntl, stat64, etc.) and since fread tends
to buffer data in big chunks, thus depleting the entropy pool.
svn:r1717