This was way overdue. User passwords are no longer stored as cleartext.
When PBot is restarted after applying this commit, all stored passwords will
be converted to salted hash digests.
The `useradd`, `userset` and `my` commands will now hash passwords.
Why did it take me so long to finally get around to hashing passwords
properly, you might ask. The reason why this wasn't done sooner is because
all of my users used hostmask-based `autologin`. The passwords that PBot
randomly generated were ignored and never used.
I do regret that it took me so long to get around to this, for those of you
who might be using custom passwords instead of hostmask-based `autologin`.
