Cannot login in after running freebsd-update install

[Mateusz Piotrowski]

Hello,

I just run `freebsd-update fetch && freebsd-update install`. Everything went 
rather smoothly except one error message which I ignored (I guess it was 

    cap_mkdb: potential reference loop detected

but I am not sure). Unfortunately, I was unable to `login`, `su` and `sudo`. 
I was getting log messages like

    login_getclass:  'tc=' reference loop 'root'
    pam_acct_mgmt: error in service module

I rebooted and booted in single user mode (since I couldn't log in when I 
started in multiuser mode). I tried to run

     cap_mkdb /etc/login.conf

but I was getting the "cap_mkdb: potential reference loop detected" every time.

Apart from that I ran `mergemaster` and `freebsd-update rollback` but it didn't 
solve the issue.

Finally, I fixed the issue by removing  ":tc=default:" from the default login 
class in /etc/login.conf and running `cap_mkdb /etc/login.conf` afterwards.

Why did `freebsd-update install` broke /etc/login.conf? Is there anything I did
wrong/could have done better?

Cheers,

-mateusz