fix: use EVP_DecryptUpdate while decrypting

fix: use EVP_DecryptUpdate while decrypting

With OpenSSL versions 1.0.2r & 1.1.1b there were changes in regards to
how OpenSSL treats misuse of Encrypt/Decrypt EVP methods in the opposite
case. E.g. using the encrypt methods in a decrypt context. OpenSSL now
returns an error in these situations. [1]
Since tinc used the EVP_EncryptUpdate function in the cipher_decrypt
function the new sanity check was triggered causing tinc to be unusable
with said OpenSSL versions.

[1] https://github.com/openssl/openssl/pull/7852