Fix incorrect declaration of randombytes() in tweetnacl.c

[alexisrozhkov]

Declaring randombytes() with incorrect signature (u64 for length in declaration vs unsigned int in definition) leads to incorrect type casting on some platforms (armeabi-v7a for instance, probably other 32-bit platforms as well), and as a result fread() from /dev/urandom might (depends on casting result) fail trying to read more than 0x7ffff000 bytes (see notes section here).

Also it might make more sense to swap 2nd and 3rd arguments to fread (size and nmemb) and check for the number of read elements. Current implementation should also work, so it's left as is.