Allow to optionally use randombytes_stir() instead of sodium_init()

According to sonOfRa sodium_init() has some timing issues on Android.
libsodium people said randombytes_stir() can be used instead:
https://github.com/jedisct1/libsodium/issues/121

sodium_init() stays the default, randombytes_stir() can be enabled by
passing --enable-randombytes-stir to the configure script.