htpasswd: use crypt_newhash instead of the bcrypt API

htpasswd: use crypt_newhash instead of the bcrypt API

man bcrypt states:
     These functions are deprecated in favor of crypt_checkpass(3) and
     crypt_newhash(3).

hence with this change we move htpasswd to the new API, while here
also change the rounds from a hardcoded 8 to automatic selection based
on system performance.

OK florian@