[Dovecot] POP3 with mdbox

[Patrick Domack]

I converted all my box's over to mdbox, from maildir and ran into this  
issue,  I knew exactly what it was and fixed it, but might want to log  
a config error or something for others to notice the issue.

I was running with pop3_uidl_format = %f, and well, that doesn't work  
with mdbox very well, and when a user ran a UIDL command, it errored  
the user out.

Simple fix, but would probably be nicer to log a config error when  
started dovecot instead.