ftpserver.pl: Corrected misaligned indentation in POP3 handlers

Fixed incorrect indentation used in both the RETR_pop3 and LIST_pop3
functions which was 5 and 9 characters rather than 4 and 8.
ftpserver.pl: Reordered the POP3 handlers to be alphabetical

In preparation for additional POP3 tests, re-ordered the command
function defintions to be sorted alphabetically.