Implement DH-AES encrypted password scheme.

This is superior to DH-BLOWFISH as Blowfish may suffer from certain
classes of weak keys, which is difficult to mitigate against without
regenerating DH parameters repeatedly. AES also has faced far more
scrutiny and is believed to be more secure.

Reference implementation (services-side):
https://github.com/atheme/atheme/blob/master/modules/saslserv/dh-aes.c