libratbox/crypt: fix difference from glibc in sha256_crypt()

rb_crypt() was generating different SHA256 ($5$) hashes than glibc, making hashes generated with charybdis unusable in ratbox and other software, and vice versa. This was likely a mistake made when adapting the glibc code.

This commit breaks compatibility with old hashes instead. I'm not sure what would be the best way to avoid that – a different salt prefix, getenv(), or just force everyone to create new hashes. (The latter would cause trouble for people trying to share configs between charybdis 3.4 and 3.5…)