This makes a difference with mingw builds on Wine, where the subsequent
fread() may abort early in the default text mode.
The Microsoft C docs say:
"In text mode, CTRL+Z is interpreted as an EOF character on input."
Identifiers starting with an underscore and followed immediately by a capital letter are reserved by the C++ standard.
The only header guards not fixed are those in the headers auto-generated from java.
This commit adds functions:
* secp256k1_rand_bits, which works like secp256k1_rand32, but consumes
less randomness
* secp256k1_rand_int, which produces a uniform integer over any range
* secp256k1_rand_bytes_test, which works like secp256k1_rand256_test
but for arbitrary byte array
