Fixed circular dependency between DHT and net_crypto: DHT no longer
depends on net_crypto.
Moved the crypto request packets functions to crypto core and DHT.
Cleaned up/added some defines that can be used to get the true maximum
length of things like the friends request message.
MAX_DATA_SIZE has been replaced in most places by more appropriate defines.
Moved Bunch of functions from net_crypto to crypto_core.
decrypt_data_fast and decrypt_data_symmetric were the same thing
therefore, removed decrypt_data_fast.
Replaced all the crypto_secretbox_* defines with the equivalent
crypto_box_* one.
New define: crypto_box_KEYBYTES that is equal to
crypto_box_BEFORENMBYTES.
While doing the checks configure might generate "core" files and will
then try to remove them. Having a "core" directory generates an error
while runing the configure script.
There's no workaround but to rename the core directory.
