lib-smtp: address: Consistently allow address values to be NULL. If the address pointer is NULL or the localpart field is NULL or empty, it signifies that it is the NULL address "<>". In the case of smtp_address_equals(), this also likely fixes crashes at places where it may be used to compare "<>".

lib-smtp: address: Rename 'poo' to 'pool' in declaration of smtp_address_detail_parse().

lib-smtp: address: Created SMTP address detail parsing function from message_detail_address_parse().

lib-smtp: Implemented parsing, manipulation and encoding of SMTP addresses.