Make password salt generation algo-agnostic and isolate bcrypt-specific logic

[narfbg]

• Fixes a bug where the libargon2 implementation uses less entropy than intended (still safe, but below baseline).
• Aligned bcrypt salt encoding more closely to bcrypt's variant. Previously, it relied on php_crypt() doing the error correction. This is still not entirely following bcrypt as designed, but that's for another time.
• Stricter bcrypt sanity checks.