pop3: Updated the coding style of pop3_state_servergreet_resp()

Updated the coding style, in this function, to be consistent with other
response functions rather then performing a hard return on failure.
smtp: Updated the coding style of smtp_state_servergreet_resp()

Updated the coding style, in this function, to be consistant with other
response functions rather then performing a hard return on failure.
pop3: Added missing comment for pop3_state_apop_resp()